1105: [WIP] Implement ra_mbe meta variables support r=matklad a=edwin0cheng
This PR implements the following meta variable support in `ra_mba` crate (issue #720):
- [x] `path`
- [ ] `expr`
- [ ] `ty`
- [ ] `pat`
- [ ] `stmt`
- [ ] `block`
- [ ] `meta`
- [ ] `item`
*Implementation Details*
In the macro expanding lhs phase, if we see a meta variable type, we try to create a `tt:TokenTree` from the remaining input. And then we use a special set of `ra_parser` to parse it to `SyntaxNode`.
Co-authored-by: Edwin Cheng <edwin0cheng@gmail.com>
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