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/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")