De-deduplicate completion items + improve sorting

[feltech]

• Remove duplicate completion items. This seems to happen a lot with cquery (C++) completions.
• Continue to sort abbreviations below full matches.
• Sort matching filterText/labels so they appear next to each other.
• Otherwise use sortText given by the language server.

[feltech]

The main motivation for this PR was to remove the duplicates coming back from cquery.

However, I found that the sort order of results was odd (which makes filtering duplicates more expensive, and just looks strange). Perhaps they make some sense straight from the language server, but Oni then filters as the user types, so retaining the sort order from the language server makes even less sense.

Hence why I thought it reasonable to "fix" the sort order for filtered completions.

[codecov[bot]]

Codecov Report

Merging #2638 into master will increase coverage by 0.07%. The diff coverage is 88.23%.

@@            Coverage Diff             @@
##           master    #2638      +/-   ##
==========================================
+ Coverage   45.43%   45.51%   +0.07%     
==========================================
  Files         361      361              
  Lines       14563    14591      +28     
  Branches     1912     1919       +7     
==========================================
+ Hits         6617     6641      +24     
- Misses       7721     7723       +2     
- Partials      225      227       +2

Impacted Files	Coverage Δ
...ser/src/Services/Completion/CompletionSelectors.ts	76.56% <88.23%> (+7.11%)	:arrow_up:

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