fix(scope-manager): add const as a global type variable

[bradzacher]

Handles part of #2477

Unrelated, but I cleaned up some useless cruft from the lib at the same time.

[typescript-eslint]

Thanks for the PR, @bradzacher!

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[codecov]

Codecov Report

Merging #2499 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2499   +/-   ##
=======================================
  Coverage   92.80%   92.81%           
=======================================
  Files         290      290           
  Lines        9429     9430    +1     
  Branches     2638     2638           
=======================================
+ Hits         8751     8752    +1     
  Misses        323      323           
  Partials      355      355           

Flag	Coverage Δ
#unittest	92.81% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
packages/scope-manager/src/lib/dom.iterable.ts	100.00% <ø> (ø)
packages/scope-manager/src/lib/dom.ts	100.00% <ø> (ø)
...ackages/scope-manager/src/lib/es2015.collection.ts	100.00% <ø> (ø)
packages/scope-manager/src/lib/es2015.core.ts	100.00% <ø> (ø)
packages/scope-manager/src/lib/es2015.generator.ts	100.00% <ø> (ø)
packages/scope-manager/src/lib/es2015.iterable.ts	100.00% <ø> (ø)
packages/scope-manager/src/lib/es2015.promise.ts	100.00% <ø> (ø)
packages/scope-manager/src/lib/es2015.proxy.ts	100.00% <ø> (ø)
packages/scope-manager/src/lib/es2015.reflect.ts	100.00% <ø> (ø)
packages/scope-manager/src/lib/es2015.symbol.ts	100.00% <ø> (ø)
... and 32 more