Merge #1439

1439: Rich mapping of cargo watch output r=matklad a=etaoins

Currently we depend on the ASCII rendering string that `rustc` provides to populate Visual Studio Code's diagnostic. This has a number of shortcomings:

1. It's not a very good use of space in the error list
2. We can't jump to secondary spans (e.g. where a called function is defined)
3. We can't use Code Actions aka Quick Fix

This moves all of the low-level parsing and mapping to a `rust_diagnostics.ts`. This uses some heuristics to map Rust diagnostics to VsCode:

1. As before, the Rust diagnostic message and primary span is used for the root diagnostic. However, we now just use the message instead of the rendered version.

2. Every secondary span is converted to "related information". This  shows as child in the error list and can be jumped to.

3. Every child diagnostic is categorised in to three buckets:
    1. If they have no span they're treated as another line of the root messages
    2. If they have replacement text they're treated as a Code Action
    3. If they have a span but no replacement text they're treated as related information (same as secondary spans).

Co-authored-by: Ryan Cumming <[email protected]>

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