feat(eslint-plugin): Catch unused React import with new JSX transform

[lexanth]

Fixes #3303

Using null for this is consistent with the jsxFragmentPragma, but is a bit strange. However, I think it does match the intention of the new JSX transform - pushing it into the build system so that it is invisible in source code (so null is reasonable).

[nx-cloud]

Nx Cloud Report

CI ran the following commands for commit 3ee3880. Click to see the status, the terminal output, and the build insights.

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Status	Command
	nx run-many --target=build --all --parallel
	nx run-many --target=typecheck --all --parallel
	nx test @typescript-eslint/eslint-plugin
	nx test @typescript-eslint/eslint-plugin-internal
	nx test @typescript-eslint/eslint-plugin-tslint
	nx test @typescript-eslint/experimental-utils
	nx test @typescript-eslint/parser
	nx test @typescript-eslint/scope-manager
	nx test @typescript-eslint/typescript-estree
	nx test @typescript-eslint/visitor-keys

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[bradzacher]

LGTM - thanks for doing this!

[codecov]

Codecov Report

Merging #3577 (3ee3880) into master (3ef5267) will increase coverage by 0.12%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #3577      +/-   ##
==========================================
+ Coverage   92.52%   92.64%   +0.12%     
==========================================
  Files         188      327     +139     
  Lines        8464    11335    +2871     
  Branches     2602     3196     +594     
==========================================
+ Hits         7831    10501    +2670     
- Misses        263      370     +107     
- Partials      370      464      +94     

Flag	Coverage Δ
unittest	92.64% <50.00%> (+0.12%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
packages/scope-manager/src/analyze.ts	66.66% <50.00%> (ø)
...ackages/scope-manager/src/referencer/Referencer.ts	93.90% <50.00%> (ø)
packages/scope-manager/src/lib/es2019.symbol.ts	100.00% <0.00%> (ø)
...ges/scope-manager/src/definition/DefinitionBase.ts	100.00% <0.00%> (ø)
...ages/scope-manager/src/lib/esnext.asynciterable.ts	100.00% <0.00%> (ø)
packages/scope-manager/src/lib/esnext.promise.ts	100.00% <0.00%> (ø)
packages/scope-manager/src/lib/es2021.weakref.ts	100.00% <0.00%> (ø)
packages/scope-manager/src/lib/es2015.proxy.ts	100.00% <0.00%> (ø)
packages/scope-manager/src/lib/esnext.array.ts	100.00% <0.00%> (ø)
... and 131 more