Unit test for First* and Last* enums

[osyrisrblx]

https://github.com/roblox-ts/luau-ast/blob/master/src/LuauAST/impl/enums.ts#L51-L58

It's easy to accidentally break these. So we should add a quick unit test which verifies:

assert(SyntaxKind.FirstIndexableExpression === 0);
assert(SyntaxKind.FirstExpression === 0);
// TODO: verify SyntaxKind.LastIndexableExpression?
assert(SyntaxKind.LastExpression + 1 === SyntaxKind.FirstStatement);
assert(SyntaxKind.LastStatement + 1 === SyntaxKind.FirstField);
assert(SyntaxKind.LastField === Math.max(...Object.values(SyntaxKind).filter((v): v is number => typeof v === "number")));

playground

[osyrisrblx]

A way to possibly verify LastIndexableExpression:

assert(SyntaxKind.LastIndexableExpression + 1 === SyntaxKind.None);

None is pretty unlikely to ever not be the first non-indexable expression.