Implemented SQL simple enum string literal parsing

[egormanga]

Description of Changes

Ditto #2843.

enum TestKind {
	OptionA,
	OptionB,
}

#[table(name=test)]
struct Test {
	kind: TestKind,
}

SELECT * FROM test WHERE kind = 'OptionB';

This PR implements simple enum support for = and != operators (subject to further expansion).

Note: still doesn't work with enum_stringify case conversion OOTB. No clue how to implement that yet.

API and ABI breaking changes

None.

Expected complexity level and risk

1.

Testing

• Manual testing using custom enum columns.
• Needs unit tests. I leave this open for contributions or as an exercise to the employees.