docs(typescript-estree): document that duplicate filenames are unsupported

[bradzacher]

Closes #955

I also added types for something because I hated seeing the anys.
Added a vscode launch option that I used to debug this issue.

[typescript-eslint]

Thanks for the PR, @bradzacher!

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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.

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[codecov]

Codecov Report

Merging #957 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #957      +/-   ##
==========================================
- Coverage   94.08%   94.08%   -0.01%     
==========================================
  Files         114      114              
  Lines        4957     4956       -1     
  Branches     1382     1382              
==========================================
- Hits         4664     4663       -1     
  Misses        166      166              
  Partials      127      127

Impacted Files	Coverage Δ
packages/typescript-estree/src/tsconfig-parser.ts	87.83% <100%> (-0.17%)	⬇️