fix(typescript-estree): forward compat for new compound assignment operators

[rbuckton]

This changes typescript-estree to use TypeScript API's for some of the functionality in node-utils.ts so that it will be forward compatible with TypeScript 4.0's support for ES2020's ||=, &&=, and ??= compound assignment operators.

[typescript-eslint]

Thanks for the PR, @rbuckton!

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The core maintainers work on this in their personal time, so please understand that it may not be possible for them to review your work immediately.

Thanks again!

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[rbuckton]

Note that something like this is needed so that the TypeScript project can start using ??=. Without this, eslint rules like preferConst don't consider ??=, ||=, or &&= an assignment and result in false positives.

[codecov]

Codecov Report

Merging #2253 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2253      +/-   ##
==========================================
+ Coverage   93.44%   93.46%   +0.01%     
==========================================
  Files         174      174              
  Lines        7954     7953       -1     
  Branches     2282     2282              
==========================================
  Hits         7433     7433              
  Misses        246      246              
+ Partials      275      274       -1     

Flag	Coverage Δ
#unittest	93.46% <100.00%> (+0.01%)	⬆️

Impacted Files	Coverage Δ
packages/typescript-estree/src/node-utils.ts	96.73% <100.00%> (+0.52%)	⬆️