Fix autocomplete prefix for custom identifierRegexes

[AprilArcus]

Hi, Ace team! My team has been using a fork of Ace, and I'm hoping to merge some of our changes upstream so that we can return to using your official releases.

The engineer who originally accomplished this work has long since departed my team, so forgive me for this somewhat rote transcription of his working notes.

This was sent upstream in #2352, but closed because fcebd0f seemed to address the same issue. That fix unfortunately does not work as we need, and fails the behavior described in our test named test leading @ not duplicated on autocomplete

The root cause: Autocomplete prefix can be wrong for completions with a custom identifierRegex because Autocompleter.base is computed relative to a prefix computed for the default identifierRegex.

Below, I reproduce the test my colleague referred to.

'test leading @ not duplicated on autocomplete': function (test) {
    editor.setValue('');
    editor.navigateFileStart();

    var text = 'view: users { derived_table: { sql: @{;; } }';
    exec('insertstring', 1, text);
    editor.moveCursorTo(0, 38);
    editor.getValue();

    attachedSymbolsToSession(text, editor.session, { const: 'value' });
    editor.execCommand('startAutocomplete');

    var completion = editor.completer.completions.filtered.filter(function(completion) { return completion.caption === '@{const}'; });
    test.ok(completion[0]);

    editor.completer.insertMatch(completion[0]);
    test.equal(editor.getValue(), 'view: users { derived_table: { sql: @{const};; } }');

    test.done();
},

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[andrewnester]

@nightwing could you please take a look at the PR?

[andrewnester]

@AprilArcus could you please address failing build? Seems it's just related to missing semicolon

[AprilArcus]

done

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.85%. Comparing base (ff3dd69) to head (e4e7582).
Report is 768 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4919      +/-   ##
==========================================
+ Coverage   85.75%   85.85%   +0.10%     
==========================================
  Files         539      542       +3     
  Lines       40728    41138     +410     
  Branches     6445     6515      +70     
==========================================
+ Hits        34927    35320     +393     
- Misses       5801     5818      +17     

Flag	Coverage Δ
unittests	85.85% <100.00%> (+0.10%)	⬆️

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[AprilArcus]

Can anyone help me rebase this over #5084? I'm stumped.

@azmkercso @andrewnester