feat: add null literal token and compiler support with keyword parsing and test cases

Introduce TOKEN_NULL to the token enum and register it as a prefix parse rule that emits CODE_NULL. Implement the null() compiler function to emit the null constant. Extend readName() to recognize the "null" keyword and map it to TOKEN_NULL. Add test files verifying null output, and that null, true, and false cannot be used as variable names.

test/cannot_use_false_as_var.wren

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+var false = "value" // expect error

test/cannot_use_null_as_var.wren

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+var null = "value" // expect error

test/cannot_use_true_as_var.wren

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+var true = "value" // expect error

test/null.wren

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+io.write(null) // expect: null