## Summary
Introduce a `StringLiteralType` with corresponding `Display` type and a
relatively basic test that the resulting representation is as expected.
Note: we currently always allocate for `StringLiteral` types. This may
end up being a perf issue later, at which point we may want to look at
other ways of representing `value` here, i.e. with some kind of smarter
string structure which can reuse types. That is most likely to show up
with e.g. concatenation.
Contributes to #12701.
## Test Plan
Added a test for individual strings with both single and double quotes
as well as concatenated strings with both forms.
c4d628cc4c