Framework: Reinstate stricter ESLint rules

[aduth]

Related: #3187

This pull request seeks to reinstate a handful of ESLint rules which were turned off as a result of ESLint dependency upgrades in #3187. Specifically, the WordPress ESLint configuration no longer enforces the following rules after WordPress-Coding-Standards/eslint-config-wordpress@b2688d6:

• eqeqeq
• no-undef
• no-caller
• no-unused-expressions
• no-unused-vars
• no-eq-null
• one-var

For all but the last of these, we want enforcement, so these have been reinstated here. In my testing, no-eq-null is redundant with eqeqeq.

Testing instructions:

Verify that there are no lint issues:

npm run lint

cc @ntwb

[codecov]

Codecov Report

Merging #3206 into master will decrease coverage by 0.17%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #3206      +/-   ##
==========================================
- Coverage   31.43%   31.26%   -0.18%     
==========================================
  Files         222      222              
  Lines        6333     6369      +36     
  Branches     1125     1134       +9     
==========================================
  Hits         1991     1991              
- Misses       3650     3677      +27     
- Partials      692      701       +9

Impacted Files	Coverage Δ
editor/modes/visual-editor/block-list.js	0% <0%> (ø)	⬆️
editor/sidebar/index.js	0% <0%> (ø)	⬆️
editor/layout/index.js	0% <0%> (ø)	⬆️
editor/header/index.js	0% <0%> (ø)	⬆️
editor/modes/text-editor/index.js	0% <0%> (ø)	⬆️
editor/modes/visual-editor/index.js	0% <0%> (ø)	⬆️

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[aduth]

Missed adding back no-undef, committed to master in b548a83.