Combine the completion.json and functions.json editor support files into a single editor.json file

[rowanseymour]

Does #991. Still outputs the original files as well for backward compatibility

{
  "context": { ... what used to be in completion.json ..   }
  "functions": { ... what used to be in functions.json ..   }
}

[codecov]

Codecov Report

Merging #995 (10ca2f3) into master (3b9019a) will decrease coverage by 0.09%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master     #995      +/-   ##
==========================================
- Coverage   86.10%   86.01%   -0.10%     
==========================================
  Files         235      234       -1     
  Lines       10305    10322      +17     
==========================================
+ Hits         8873     8878       +5     
- Misses       1020     1026       +6     
- Partials      412      418       +6     

Impacted Files	Coverage Δ
cmd/docgen/docs/editor_support.go	75.00% <75.00%> (ø)

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