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.

test/language/map/bad_key_precedence.wren

@@ -0,0 +1,3 @@
+var map = {
+  1 + 2: "bad key" // expect error
+}