feat: allow call and unary expressions as map keys in compiler

Change the map key parsing precedence from PREC_PRIMARY to PREC_UNARY in
wren_compiler.c, enabling method calls (e.g., name.count), subscript
access (name[0]), and unary operators (-1, ~3) as valid map keys.

Add two test files: precedence.wren verifying the new key types work
correctly, and bad_key_precedence.wren confirming that binary operators
like addition still produce a parse error.

src/vm/wren_compiler.c

@@ -1987,7 +1987,7 @@ static void map(Compiler* compiler, bool allowAssignment)
     if (peek(compiler) == TOKEN_RIGHT_BRACE) break;
 
     // The key.
-    parsePrecedence(compiler, false, PREC_PRIMARY);
+    parsePrecedence(compiler, false, PREC_UNARY);
     consume(compiler, TOKEN_COLON, "Expect ':' after map key.");
     ignoreNewlines(compiler);