improve snippet manager and add tests

[nightwing]

fixes #2925 #2926 #3924 #2471

[codecov]

Codecov Report

Merging #4012 into master will increase coverage by 0.33%.
The diff coverage is 87.53%.

@@            Coverage Diff             @@
##           master    #4012      +/-   ##
==========================================
+ Coverage   69.55%   69.89%   +0.33%     
==========================================
  Files         517      517              
  Lines       50224    50382     +158     
  Branches     9474     9492      +18     
==========================================
+ Hits        34933    35214     +281     
+ Misses      15291    15168     -123

Impacted Files	Coverage Δ
lib/ace/range_list.js	88.69% <100%> (+0.06%)	⬆️
lib/ace/snippets_test.js	99.36% <100%> (+1.03%)	⬆️
lib/ace/autocomplete.js	63.66% <33.33%> (-0.91%)	⬇️
lib/ace/snippets.js	78.76% <84.11%> (+20.33%)	⬆️
lib/ace/multi_select.js	76.29% <0%> (+0.34%)	⬆️

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[fjakobs]

Do we have snippets in the Ace code base that would benefit from these changes?

[fjakobs]

How would we be able to configure this value in the IDE?

[nightwing]

the snippet loader plugin https://github.com/c9/core/blob/d8917f33fd7d32141d895fb7de530cfee2a416cf/plugins/c9.ide.language.generic/snippet_loader.js#L7 will need to do

snippetManager.variables.WORKSPACE_NAME=function() { return c9.projectName }
snippetManager.variables.FILEPATH=function(editor) { return editor.session.c9doc.tab.path }

[nightwing]

Do we have snippets in the Ace code base that would benefit from these changes?

not directly, but this will allow us to replace some of broken snippets that use vim specific backtick syntax with snippets from textmate bundles that will work now.