[Proposal]: Enhanced #line directives

# Enhanced #line directives

* [x] Proposed
* [x] Prototype: Done
* [x] Implementation: Done
* [ ] Specification: Not Started

## Summary
[summary]: #summary
The compiler applies the mapping defined by `#line` directives to diagnostic locations and sequence points emitted to the PDB.

Currently only the line number and file path can be mapped while the starting character is inferred from the source code. The proposal is to allow specifying full span mapping.

`#line (startLine, startChar) - (endLine, endChar) charOffset "fileName"`

Spec: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-10.0/enhanced-line-directives.md

## Drawbacks
[drawbacks]: #drawbacks

## Alternatives
[alternatives]: #alternatives

## Unresolved questions
[unresolved]: #unresolved-questions

## Design meetings