fix: make sure that custom parser is used if passed to process

[zth]

This ensures that a custom parser is used if passed to the server.

[codecov]

Codecov Report

Merging #1438 into master will increase coverage by 0.01%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master    #1438      +/-   ##
==========================================
+ Coverage   58.96%   58.98%   +0.01%     
==========================================
  Files          60       60              
  Lines        3022     3023       +1     
  Branches      796      796              
==========================================
+ Hits         1782     1783       +1     
  Misses       1194     1194              
  Partials       46       46

Impacted Files	Coverage Δ
...raphql-language-service-server/src/GraphQLCache.ts	51.82% <100%> (+0.17%)	⬆️
...ql-language-service-server/src/MessageProcessor.ts	57.4% <75%> (ø)	⬆️

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[acao]

@zth looks great!
maybe we should add a test for this?

[zth]

I think tests would be great for sure, but there are no tests at that level right now from what I can see, and I think it'd be a bit involved to add it. Seeing as this fixes a feature that's currently broken, could we have this merged before tests are added?