Sort diagnostics according to severity for detailed diag responses

[bstaletic]

Users are more likely to be interested in errors than warnings in /detailed_diagnostic responses. That means we need to sort the diagnostics before finding the one nearest to the cursor.

For LSP completers, that's easy as the severity is numerical. For Omnisharp-roslyn and TSServer, the severity is a word and we can't just say sort( key = severity ).

Since LSP severity was convenient for comparison, I have opted for sorting C# and TS diags by the equivalent LSP severity.

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This change is 

[codecov[bot]]

Codecov Report

Merging #1728 (5fb0927) into master (0ce532e) will increase coverage by 0.40%. The diff coverage is 66.66%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1728 +/- ## ========================================== + Coverage 95.05% 95.46% +0.40% ========================================== Files 51 83 +32 Lines 6711 8194 +1483 Branches 0 163 +163 ========================================== + Hits 6379 7822 +1443 + Misses 332 322 -10 - Partials 0 50 +50 ```

[bstaletic]

According to the coverage data, we're only ever dealing with errors in our tests, at least for C# and TS.

[puremourning]

Don't we just need some code which produces a warning and error on the same line.

I think clangd does this. maybe something like:

int x;
if ( x = 2 )
{
}

With -Wno-error -Werror=uninitialized

[mergify[bot]]

Thanks for sending a PR!