Added new LooseAutocomplete type

[alexiglesias93]

LooseAutocomplete

A type that allows for variable autocompletion with loose validation.

Example:

const f = (foo: LooseAutocomplete<'a' | 'b'>) => foo;
f(''); // VSCode autocomplete shows 'a' and 'b' as options, but doesn't fail when a different option is provided.
f('a'); // Valid
f('b'); // Valid
f('c'); // Valid