Add canvas context check in $updateDecorators method

[mkslanc]

Issue #, if available:

Description of changes:
Guard check for environments where canvas doesn't have getContext method

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Pull Request Checklist:

• No backwards incompatible changes were made to Ace's public interface which is defined through the main typings file (ace.d.ts) and its references:
  • https://github.com/ajaxorg/ace/blob/master/ace.d.ts
  • https://github.com/ajaxorg/ace/blob/master/ace-modes.d.ts
  • https://github.com/ajaxorg/ace/blob/master/ace-extensions.d.ts

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 87.73%. Comparing base (017341f) to head (71e6ddb).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/ext/diff/scroll_diff_decorator.js	50.00%	1 Missing ⚠️
src/layer/decorators.js	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5833      +/-   ##
==========================================
- Coverage   87.74%   87.73%   -0.01%     
==========================================
  Files         618      618              
  Lines       46042    46046       +4     
  Branches     7547     7549       +2     
==========================================
+ Hits        40398    40400       +2     
- Misses       5644     5646       +2     

Flag	Coverage Δ
unittests	87.73% <50.00%> (-0.01%)	⬇️

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