fix(typescript-estree): handle BigInt with _ numeric separator

[fisker]

Fix BigInt with numberic separator parsing.

The estree spec only said n should not include in bigint, so I guess _ should perserve.

Update: The spec says bigint property is the string representation of the BigInt value, and we are using String(BigInt('123')) for 1_2_3n, it doesn't includes _, so it should not include _.

[typescript-eslint]

Thanks for the PR, @fisker!

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[codecov]

Codecov Report

Merging #2067 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2067   +/-   ##
=======================================
  Coverage   93.40%   93.40%           
=======================================
  Files         171      171           
  Lines        7745     7745           
  Branches     2210     2210           
=======================================
  Hits         7234     7234           
  Misses        246      246           
  Partials      265      265           

Flag	Coverage Δ
#unittest	93.40% <100.00%> (ø)

Impacted Files	Coverage Δ
packages/typescript-estree/src/convert.ts	98.88% <100.00%> (ø)

[bradzacher]

LGTM - thanks for finding and fixing this!