refactor: split source tree into vm/ and cli/ directories with updated build system

Separate Wren VM library sources from CLI tool sources by moving them into
src/vm/ and src/cli/ subdirectories respectively. Update the Makefile to use
separate wildcard patterns for each directory, generate distinct object file
paths under build/vm/ and build/cli/, and adjust include paths to src/include.
Also update the Xcode project file to reference the new file locations.

src/cli/main.c

@@ -0,0 +1,193 @@
+#include <stdarg.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "wren.h"
+
+#define MAX_LINE_LENGTH 1024 // TODO: Something less arbitrary.
+#define MAX_PATH_LENGTH 2024 // TODO: Something less arbitrary.
+
+// This is the source file for the standalone command line interpreter. It is
+// not needed if you are embedding Wren in an application.
+
+char rootDirectory[MAX_PATH_LENGTH];
+
+static void failIf(bool condition, int exitCode, const char* format, ...)
+{
+  if (!condition) return;
+
+  va_list args;
+  va_start(args, format);
+  vfprintf(stderr, format, args);
+  va_end(args);
+
+  exit(exitCode);
+}
+
+static char* readFile(const char* path)
+{
+  FILE* file = fopen(path, "rb");
+  failIf(file == NULL, 66, "Could not open file \"%s\".\n", path);
+
+  // Find out how big the file is.
+  fseek(file, 0L, SEEK_END);
+  size_t fileSize = ftell(file);
+  rewind(file);
+
+  // Allocate a buffer for it.
+  char* buffer = (char*)malloc(fileSize + 1);
+  failIf(buffer == NULL, 74, "Could not read file \"%s\".\n", path);
+
+  // Read the entire file.
+  size_t bytesRead = fread(buffer, sizeof(char), fileSize, file);
+  failIf(bytesRead < fileSize, 74, "Could not read file \"%s\".\n", path);
+
+  // Terminate the string.
+  buffer[bytesRead] = '\0';
+
+  fclose(file);
+  return buffer;
+}
+
+static char* readModule(WrenVM* vm, const char* module)
+{
+  // The module path is relative to the root directory and with ".wren".
+  size_t rootLength = strlen(rootDirectory);
+  size_t moduleLength = strlen(module);
+  size_t pathLength = rootLength + moduleLength + 5;
+  char* path = (char*)malloc(pathLength + 1);
+  memcpy(path, rootDirectory, rootLength);
+  memcpy(path + rootLength, module, moduleLength);
+  memcpy(path + rootLength + moduleLength, ".wren", 5);
+  path[pathLength] = '\0';
+
+  FILE* file = fopen(path, "rb");
+  if (file == NULL)
+  {
+    free(path);
+    return NULL;
+  }
+
+  // Find out how big the file is.
+  fseek(file, 0L, SEEK_END);
+  size_t fileSize = ftell(file);
+  rewind(file);
+
+  // Allocate a buffer for it.
+  char* buffer = (char*)malloc(fileSize + 1);
+  if (buffer == NULL)
+  {
+    fclose(file);
+    free(path);
+    return NULL;
+  }
+
+  // Read the entire file.
+  size_t bytesRead = fread(buffer, sizeof(char), fileSize, file);
+  if (bytesRead < fileSize)
+  {
+    free(buffer);
+    fclose(file);
+    free(path);
+    return NULL;
+  }
+
+  // Terminate the string.
+  buffer[bytesRead] = '\0';
+
+  fclose(file);
+  free(path);
+
+  return buffer;
+}
+
+static int runFile(WrenVM* vm, const char* path)
+{
+  // Use the directory where the file is as the root to resolve imports
+  // relative to.
+  const char* lastSlash = strrchr(path, '/');
+  if (lastSlash != NULL)
+  {
+    memcpy(rootDirectory, path, lastSlash - path + 1);
+    rootDirectory[lastSlash - path + 1] = '\0';
+  }
+
+  char* source = readFile(path);
+
+  int result;
+  switch (wrenInterpret(vm, path, source))
+  {
+    case WREN_RESULT_SUCCESS: result = 0; break;
+    case WREN_RESULT_COMPILE_ERROR: result = 65; break; // EX_DATAERR.
+    case WREN_RESULT_RUNTIME_ERROR: result = 70; break; // EX_SOFTWARE.
+    default:
+      // Unreachable.
+      result = 255;
+      break;
+  }
+  
+  wrenFreeVM(vm);
+  free(source);
+
+  return result;
+}
+
+static int runRepl(WrenVM* vm)
+{
+  printf("\\\\/\"-\n");
+  printf(" \\_/   wren v0.0.0\n");
+
+  // Import relative to the current directory.
+  rootDirectory[0] = '\0';
+
+  char line[MAX_LINE_LENGTH];
+
+  for (;;)
+  {
+    printf("> ");
+
+    if (fgets(line, MAX_LINE_LENGTH, stdin))
+    {
+      // TODO: Handle failure.
+      wrenInterpret(vm, "Prompt", line);
+
+      // TODO: Automatically print the result of expressions.
+    }
+    else
+    {
+      printf("\n");
+      return 0;
+    }
+  }
+
+  wrenFreeVM(vm);
+  return 0;
+}
+
+int main(int argc, const char* argv[])
+{
+  if (argc < 1 || argc > 2)
+  {
+    fprintf(stderr, "Usage: wren [file]\n");
+    return 64; // EX_USAGE.
+  }
+
+  WrenConfiguration config;
+
+  config.loadModuleFn = readModule;
+
+  // Since we're running in a standalone process, be generous with memory.
+  config.initialHeapSize = 1024 * 1024 * 100;
+
+  // Use defaults for these.
+  config.reallocateFn = NULL;
+  config.minHeapSize = 0;
+  config.heapGrowthPercent = 0;
+
+  WrenVM* vm = wrenNewVM(&config);
+
+  if (argc == 1) return runRepl(vm);
+  if (argc == 2) return runFile(vm, argv[1]);
+}