Implement `literal_from_str` for proc macro server
Closes#16233
Todos and unanswered questions:
- [x] Is this the correct approach? Can both the legacy and `rust_analyzer_span` servers depend on the `syntax` crate?
- [ ] How should we handle suffixes for string literals? It doesn't seem like `rust-analyzer` preservers suffix information after parsing.
- [x] Why are the `expect` tests failing? Specifically `test_fn_like_macro_clone_literals`
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