feat: add Stdin.isTerminal method to detect TTY connection

Add a new `isTerminal` static method to the Stdin class that returns `true` when
stdin is connected to a TTY (interactive terminal) and `false` when input comes
from a pipe. Implementation uses libuv's `uv_guess_handle` to check the stdin
descriptor. Includes documentation, foreign method declaration, module
registration, and a test expecting `false` since tests run non-interactively.

src/module/io.c

@@ -539,6 +539,12 @@ void stdinIsRawSet(WrenVM* vm)
   }
 }
 
+void stdinIsTerminal(WrenVM* vm)
+{
+  initStdin();
+  wrenSetSlotBool(vm, 0, uv_guess_handle(stdinDescriptor) == UV_TTY);
+}
+
 static void allocCallback(uv_handle_t* handle, size_t suggestedSize,
                           uv_buf_t* buf)
 {

src/module/io.wren

@@ -217,6 +217,7 @@ foreign class Stat {
 class Stdin {
   foreign static isRaw
   foreign static isRaw=(value)
+  foreign static isTerminal
 
   static readByte() {
     return read_ {

src/module/io.wren.inc

@@ -219,6 +219,7 @@ static const char* ioModuleSource =
 "class Stdin {\n"
 "  foreign static isRaw\n"
 "  foreign static isRaw=(value)\n"
+"  foreign static isTerminal\n"
 "\n"
 "  static readByte() {\n"
 "    return read_ {\n"