Add `SrcRef` type and create srcrefs with `parse_status()`

[lionel-]

Branched from #481.

Add SrcRef type and enable parsing with srcrefs in parse_status(). We now use this in parse_exprs_with_srcrefs() instead of calling back into the R-level parse(). Also adds a bunch of tests to this previously untested area.

Will be useful to write tools that detect expression boundaries in R code. Progress towards posit-dev/positron#1326

Edit: srcref is now a submodule of parser. I'll move parse.rs and source.rs to that submodule outside of this PR.

[lionel-]

@DavisVaughan Can you take another quick look please?

• I've moved the new files to a parser submodule. I plan to move parse.rs and source.rs there outside of this PR.

• The srcfile is no longer a parse option but a variant of input. I structured things this way so that srcfile is owned by the caller, which allows it to retrieve parse data.

• I renamed line to line_virtual and line_parsed to line.

• The line fields are now [ ) ranges like the column fields. It's important not only for internal consistency but also because it's expected by methods of std::ops::Range.