feat: allow zero or multiple imported names in import statement

Make the `for` clause in import statements optional, and support comma-separated lists of imported names when present. This removes the previous restriction of exactly one imported variable per import statement.

- Refactor `import()` in compiler to handle optional `for` clause with loop over comma-separated names
- Update existing tests to use comma-separated imports and omit `for` clause where appropriate
- Add new tests: `inside_block`, `name_collision`, `no_variable`

test/module/change_imported_value/change_imported_value.wren

@@ -1,6 +1,4 @@
-// TODO: Use comma-separated list.
-import "module" for Module
-import "module" for Other
+import "module" for Module, Other
 
 IO.print(Module) // expect: before
 

test/module/cyclic_import/cyclic_import.wren

@@ -1,4 +1,4 @@
-import "a" for A
+import "a"
 
 // Shared module should only run once:
 // expect: start a

test/module/implicitly_imports_core/implicitly_imports_core.wren

@@ -1,5 +1,4 @@
-// TODO: Allow omitting "for" clause.
-import "module" for Module
+import "module"
 // expect: Bool
 // expect: Class
 // expect: Fiber

test/module/implicitly_imports_core/module.wren

@@ -1,6 +1,4 @@
 // nontest
-var Module = "from module"
-
 IO.print(Bool)
 IO.print(Class)
 IO.print(Fiber)

test/module/inside_block/inside_block.wren

@@ -0,0 +1,10 @@
+var Module = "outer"
+
+if (true) {
+  import "module" for Module
+  // expect: ran module
+
+  IO.print(Module) // expect: from module
+}
+
+IO.print(Module) // expect: outer

test/module/inside_block/module.wren

@@ -0,0 +1,3 @@
+// nontest
+var Module = "from module"
+IO.print("ran module")

test/module/multiple_variables/multiple_variables.wren

@@ -1,9 +1,4 @@
-// TODO: Comma-separated list.
-import "module" for Module1
-import "module" for Module2
-import "module" for Module3
-import "module" for Module4
-import "module" for Module5
+import "module" for Module1, Module2, Module3, Module4, Module5
 
 // Only execute module body once:
 // expect: ran module

test/module/name_collision.wren

@@ -0,0 +1,2 @@
+var Collides
+import "module" for Collides // expect error

test/module/no_variable/module.wren

@@ -0,0 +1,2 @@
+// nontest
+IO.print("ran module")

test/module/no_variable/no_variable.wren

@@ -0,0 +1,2 @@
+import "module"
+// expect: ran module