Only return InsertTextFormat.Snippet when a snippet is returned

[tomv564]

The language server was returning InsertTextFormat.Snippet all the time when a client indicated it supported snippets.

The more correct behaviour is to only return that value when insertText is actually a snippet.
This allows clients to avoid potentially more complex completion handling when not necessary.

[codecov]

Codecov Report

Merging #363 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #363      +/-   ##
==========================================
- Coverage   74.71%   74.68%   -0.03%     
==========================================
  Files          15       15              
  Lines        1760     1758       -2     
  Branches      332      331       -1     
==========================================
- Hits         1315     1313       -2     
  Misses        294      294              
  Partials      151      151

Impacted Files	Coverage Δ
src/typescript-service.ts	72.96% <100%> (-0.11%)	⬇️

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