feat(types): add type predicate to isLoaded

[lukeKonopka]

Description

We know that if isLoaded() helper returns true the argument is not undefined. I added an overload to isLoaded function type so that it will narrow type of its argument.

Unfortunately this only works for single argument (we can't have type predicates that reference spread arguments).

Check List

If not relevant to pull request, check off as complete

• [x] All tests passing
• [x] Docs updated with any changes or examples if applicable
• [x] Added tests to ensure new feature(s) work properly

Relevant Issues

Fixes #955

[codecov[bot]]

Codecov Report

Merging #956 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #956   +/-   ##
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  Coverage   88.33%   88.33%           
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  Files          29       29           
  Lines         797      797           
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  Hits          704      704           
  Misses         93       93