feat(spellcheckerprovider): async spellchecker support

[kwonoj]

closes #218

This PR implements feature to support async spellchecker interface from electron@5.

PR doesn't include any backward interface compatibility, makes only supports electron@5 and above.

Notable breaking changes are

• spellCheckerProvider now having all async interfaces only to support proxy interop easily
• spellCheckerProvider no longer attach to webFrame directly, it's now separate jobs for attachSpellCheckProvider
• loadDictionary no longer accepts file path to access file directly
• verboselog is removed, barely used

api surface has bit expanded by separating provider to attach into webframe. But this allows to instantiate provider in other thread / process, achieves true async spellchecker without blocking main thread. For simplicity, attachProvider accepts provider as-is if it's to be used in main thread.

[codecov]

Codecov Report

Merging #389 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #389   +/-   ##
======================================
  Coverage    99.3%   99.3%           
======================================
  Files           3       3           
  Lines         143     143           
  Branches       25      25           
======================================
  Hits          142     142           
  Partials        1       1

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[codecov]

Codecov Report

Merging #389 into master will decrease coverage by 7.19%.
The diff coverage is 88.73%.

@@           Coverage Diff            @@
##           master    #389     +/-   ##
========================================
- Coverage    99.3%   92.1%   -7.2%     
========================================
  Files           3       4      +1     
  Lines         143     114     -29     
  Branches       25      17      -8     
========================================
- Hits          142     105     -37     
- Misses          0       6      +6     
- Partials        1       3      +2

Impacted Files	Coverage Δ
src/index.ts	100% <100%> (ø)	⬆️
src/attachSpellCheckProvider.ts	80.95% <80.95%> (ø)
src/spellCheckerProvider.ts	93.97% <91.83%> (-5.28%)	⬇️

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