Completion improvements: workspace symbols & executables

[skovhus]

Completion handler improvements:

• symbols from other files are now also used (both variables and functions for now – we could consider limiting this), but we prefer local symbols if we have duplicates
• symbol completions from other files utilize the documentation feature to show where the symbol comes from
• remove duplicated completions for executables and builtins (e.g. echo is both a builtin AND have a man page)
• ensure that executable programs on the paths are actually executable

[codecov]

Codecov Report

Merging #215 into master will increase coverage by 1.49%.
The diff coverage is 64.86%.

@@            Coverage Diff             @@
##           master     #215      +/-   ##
==========================================
+ Coverage   70.16%   71.65%   +1.49%     
==========================================
  Files          18       18              
  Lines         476      494      +18     
  Branches       68       71       +3     
==========================================
+ Hits          334      354      +20     
+ Misses        125      122       -3     
- Partials       17       18       +1     

Impacted Files	Coverage Δ
server/src/util/fs.ts	75.00% <ø> (+1.31%)	⬆️
server/src/server.ts	57.14% <52.17%> (-3.07%)	⬇️
server/src/analyser.ts	83.33% <85.71%> (+9.91%)	⬆️
server/src/executables.ts	88.88% <85.71%> (+13.13%)	⬆️
server/src/util/array.ts	83.33% <0.00%> (-16.67%)	⬇️

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