feat: better typing on filterMap for use with predicates

[wschurman]

Why

When a map is filtered with a predicate, if the predicate signals a type using the is operator, then the resulting map should be have values of the type of the predicate result. For instance, if the type Map<string, string | null> is filtered with a truthy predicate, then the return type of filterMap should be Map<string, string>.

Array accomplishes this with a function declaration overload: https://github.com/microsoft/TypeScript/blob/master/lib/lib.es5.d.ts#L1176

How

Add a function declaration overload.

Test Plan

yarn tsc, also run new test.

[codecov]

Codecov Report

Merging #106 (5e0f697) into master (8354741) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #106   +/-   ##
=======================================
  Coverage   94.95%   94.95%           
=======================================
  Files          66       66           
  Lines        1624     1624           
  Branches      195      195           
=======================================
  Hits         1542     1542           
  Misses         80       80           
  Partials        2        2           

Flag	Coverage Δ
integration	94.95% <100.00%> (ø)
unittest	94.95% <100.00%> (ø)

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Impacted Files	Coverage Δ
packages/entity/src/utils/collections/maps.ts	100.00% <100.00%> (ø)

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