feat: replace block syntax with fn keyword for function literals across compiler and tests

Rename `block` parsing rule to `fn` by adding TOKEN_FN token and function prefix rule, removing the old block prefix from TOKEN_LEFT_BRACE. Update compiler.c to replace `block()` with `function()` and add fn equality primitives (fn_eqeq, fn_bangeq) in primitives.c. Migrate test files: rename block_call.wren to fn_call.wren, block_syntax.wren to fn_syntax.wren, and replace `{ ... }` with `fn { ... }` or `fn expr` syntax. Remove obsolete bool_equality.wren test.

test/fn_call.wren

@@ -0,0 +1,3 @@
+io.write((fn 123).call) // expect: 123
+
+// TODO(bob): Arguments.