Better errors for indexing gettable/settable values

[DanielRosenwasser]

@sandersn found a case where, upon turning on noImplicitAny in Puppeteer, we issued the following error on a Map:

Element implicitly has an 'any' type because type 'Map<string, Function>' has no index signature.

We should give a more specialized error message in cases where there is no index signature, but the value has a method get that takes a string, or when

Element implicitly has an 'any' type because type '{0}' has no index signature. Did you mean to call '{1}'?

[collin5]

@DanielRosenwasser I would love to give this a try as my first PR. Any pointers to give me direction on this? Thank you!

[sandersn]

@collin5 I would start by searching for the error message that you want to replace. Spaces are replaced by underscores in the code. Then add a case above that uses getTypeOfPropertyOfType, getSingleCallSignature, getMinArgumentCount and getTypeAtPosition to look for a method named get with a single string string parameter.

Two more things:

1. diagnosticMessage.json is where you add error messages.
2. tests/cases/compiler/ is where you add new test cases, which will add new baselines after a test run.

[collin5]

@sandersn Thank you! Will open a PR for this soon.