fix(typescript-estree): ImportDeclaration.specifier to Literal

[pablobirukov]

According to the spec, ImportDeclaration.source should have type Literal

[typescript-eslint]

Thanks for the PR, @pablobirukov!

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

@@            Coverage Diff             @@
##           master     #974      +/-   ##
==========================================
- Coverage   94.13%   94.13%   -0.01%     
==========================================
  Files         115      115              
  Lines        5017     5016       -1     
  Branches     1403     1403              
==========================================
- Hits         4723     4722       -1     
  Misses        166      166              
  Partials      128      128

Impacted Files	Coverage Δ
.../eslint-plugin/src/rules/triple-slash-reference.ts	89.65% <100%> (-0.35%)	⬇️

[bradzacher]

LGTM, thanks for this