Allow undefined/null to override any compiler option

[weswigham]

In the resulting js object, the value for either null or undefined is always undefined (null is allowed in the json file, since undefined is not; however this avoids dealing with null within the compiler), and we no longer issue any errors on seeing either value while validating a config. Except "extends", since that's not really a compiler option and can't meaningfully be "overwritten".

Also fixes path handling in extended configs to be relative to the including file, as was in the original feature and implementation. This must have silently regressed at some point, as there were configs which should have tested this still present; but no tests using them. This is a bugfix, but may constitute a breaking change from the prior release, depending on how long the bug has been present. But hey, nobody complained when it changed the first time, so nobody's likely to complain when it changes back, right?

Fixes #14184

[mhegazy]

@weswigham does this also address #17617?

[weswigham]

@mhegazy It should, since isFilePath is true for baseUrl.

[ghost]

This PR breaks forceConsistentCasingInFileNames; I suspect this is due to "canonicalizing" (lowercasing) paths. You can reproduce this by running tsc in DefinitelyTyped/types/react-navigation (or most any other package there) on windows. Do you know why you normalized the paths in this PR?

[weswigham]

@andy-ms toPath was mostly just being used to get a canonical joined path for the new base path. If the canonicalization is an issue, I can adjust it to not canonicalize, as I do not believe it to be critical.