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.
