12$: warn on '} if' on same line with no 'else'

[strager]

if (x) {
} if (y) {  // warn please!
}

if (x) { } if (y) { }  // unsure if we should warn...

Inspiration: https://twitter.com/tsoding/status/1782676054093533328

[strager]

Proposed logic:

After parsing the { } body of an if, check if the next token is Token_Type::kw_if and if has_leading_newline is false. If yes, then warn.

[qnguyen1013]

Hey Strager, I'm a recent cs college grad and am trying to get into open source, would love to try and tackle this issue!

[strager]

@qnguyen1013 Sure thing! Here are our contribution guides: https://quick-lint-js.com/contribute/

If you need any help, let me know.