feat: type narrow types with literal fields

[lewis6991]

---@class A
---@field type 'a'

---@class B
---@field type 'b'

local obj --- @type A|B

if obj.type == 'a' then
    -- obj is narrowed to type A
end

Currently only narrows types defined with @class. I attempted to make this work for @alias (as in #704) but it got rather complicated.

It may be beneficial to cache the result of getNodeTypesWithLiteralField.

[sumneko]

Thank you!