fix(eslint-plugin): add isTypeAssertion utility function

[bradzacher]

While working on a new rule, I noticed that there were a few places that we didn't check for both angle bracket and as assertions.
This introduces a utility function which does that, and uses it everywhere.

[typescript-eslint]

Thanks for the PR, @bradzacher!

typescript-eslint is a 100% community driven project, and we are incredibly grateful that you are contributing to that community.

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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[codecov]

Codecov Report

Merging #1369 into master will decrease coverage by 0.03%.
The diff coverage is 88.46%.

@@            Coverage Diff             @@
##           master    #1369      +/-   ##
==========================================
- Coverage   94.05%   94.02%   -0.04%     
==========================================
  Files         133      133              
  Lines        5940     5941       +1     
  Branches     1686     1685       -1     
==========================================
- Hits         5587     5586       -1     
- Misses        186      187       +1     
- Partials      167      168       +1

Impacted Files	Coverage Δ
...-plugin/src/rules/explicit-function-return-type.ts	100% <100%> (ø)	⬆️
packages/eslint-plugin/src/util/astUtils.ts	87.5% <60%> (-7.24%)	⬇️
...ackages/eslint-plugin/src/rules/no-extra-parens.ts	96.22% <94.44%> (-0.14%)	⬇️