fix(typescript-estree): handle TS4.0 breaking change in TupleType

[uniqueiniquity]

Fixes #2098.

In TypeScript 4.0, the elementTypes property of TupleType will be renamed to elements.
Here, we update our AST conversion to handle both versions and normalize to the older name to avoid breaking existing rule implementations.

[typescript-eslint]

Thanks for the PR, @uniqueiniquity!

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[bradzacher]

thanks for this!

[codecov]

Codecov Report

Merging #2197 into master will decrease coverage by 0.02%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master    #2197      +/-   ##
==========================================
- Coverage   93.42%   93.40%   -0.03%     
==========================================
  Files         173      173              
  Lines        7862     7864       +2     
  Branches     2243     2243              
==========================================
  Hits         7345     7345              
- Misses        245      246       +1     
- Partials      272      273       +1     

Flag	Coverage Δ
#unittest	93.40% <33.33%> (-0.03%)	⬇️

Impacted Files	Coverage Δ
packages/typescript-estree/src/convert.ts	98.61% <33.33%> (-0.28%)	⬇️