fix: Cannot read property of null for bgTokenizer after session is destroyed

[andrewnester]

Issue #, if available: #4711

Description of changes:
Always set bgTokenizer when edit session is created and never set it to null. Instead use session.destroyed flag and remove all listeners from bgTokenizer.

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[codecov]

Codecov Report

Merging #4713 (57e05cd) into master (bf2c7e2) will decrease coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4713      +/-   ##
==========================================
- Coverage   71.19%   71.11%   -0.08%     
==========================================
  Files         553      554       +1     
  Lines       55598    55706     +108     
  Branches    10392    10418      +26     
==========================================
+ Hits        39584    39617      +33     
- Misses      16014    16089      +75     

Flag	Coverage Δ
unittests	71.11% <100.00%> (-0.08%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
lib/ace/background_tokenizer.js	97.08% <100.00%> (+0.18%)	⬆️
lib/ace/edit_session.js	85.68% <100.00%> (+0.17%)	⬆️
lib/ace/edit_session_test.js	98.25% <100.00%> (+0.01%)	⬆️
lib/ace/editor.js	79.66% <100.00%> (+0.01%)	⬆️
lib/ace/worker/mirror.js	51.35% <0.00%> (-9.26%)	⬇️
lib/ace/lib/dom.js	83.59% <0.00%> (-1.27%)	⬇️
lib/ace/ext/code_lens.js	91.60% <0.00%> (-0.48%)	⬇️
lib/ace/keyboard/textinput.js	75.45% <0.00%> (-0.26%)	⬇️
lib/ace/autocomplete/popup.js	78.19% <0.00%> (ø)
lib/ace/mode/php_completions.js	26.08% <0.00%> (ø)
... and 6 more

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