chore: bump typescript dependency to 3.8

[bradzacher]

This is the first step in implementing the 3.8 features.
I wanted to keep each PR small and focused, so this is the minimum required work to get the build passing with 3.8 installed.

See #1436

[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 #1696 into master will decrease coverage by 0.18%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1696      +/-   ##
==========================================
- Coverage   95.35%   95.17%   -0.19%     
==========================================
  Files         148      148              
  Lines        6931     6938       +7     
  Branches     1992     1999       +7     
==========================================
- Hits         6609     6603       -6     
- Misses        112      124      +12     
- Partials      210      211       +1

Impacted Files	Coverage Δ
packages/typescript-estree/src/convert.ts	98.86% <0%> (-0.57%)	⬇️
...pt-estree/src/create-program/createWatchProgram.ts	87.13% <0%> (-5.27%)	⬇️
...eslint-plugin/src/rules/type-annotation-spacing.ts	80% <0%> (-0.56%)	⬇️
...nt-plugin/src/rules/no-unused-vars-experimental.ts	91.48% <0%> (+0.09%)	⬆️
...ackages/eslint-plugin/src/rules/member-ordering.ts	92.68% <0%> (+0.37%)	⬆️