fix(eslint-plugin): [prefer-literal-enum-member] allow negative numbers

[bradzacher]

Fixes #2276

[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 #2277 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2277   +/-   ##
=======================================
  Coverage   93.10%   93.11%           
=======================================
  Files         283      283           
  Lines        9024     9029    +5     
  Branches     2467     2469    +2     
=======================================
+ Hits         8402     8407    +5     
  Misses        301      301           
  Partials      321      321           

Flag	Coverage Δ
#unittest	93.11% <100.00%> (+<0.01%)	⬆️

Impacted Files	Coverage Δ
...int-plugin/src/rules/prefer-literal-enum-member.ts	100.00% <100.00%> (ø)