Migrate to std::filesystem usage in FilePath

[borrrden]

This leaves the FilePath interface intact and replaces its implementation with std::filesystem. This is the cleanest way to immediately eliminate legacy code using non standard C APIs to accomplish the same. As a follow up, FilePath could be removed entirely.

[borrrden]

I forgot to lint 😢 and also Windows is unhappy with some of this for whatever reason....(seems like it doesn't have an implicit conversion like the other compilers to)

[cbl-bot]

Code Coverage Results:

Type	Percentage
branches	64.88
functions	77.22
instantiations	70.92
lines	76.07
regions	72.34

[borrrden]

There was a Windows PR validation failure but it looks like flake rather than anything related to this PR.

[snej]

Is there a way to keep the FilePath API closer to what it was so there are fewer changes in the codebase?
I have a gut feeling that std::filesystem should remain an internal detail of FilePath until we're ready to replace it entirely. But I'm open to counter-arguments :)

[borrrden]

Is there a way to keep the FilePath API closer to what it was so there are fewer changes in the codebase?

That's exactly what I did, except for the constructor (and filepath is easily convertible to and from string so think of it as a string constructor). It was really awkward to keep the dir and file separate in this new system because it adds no meaning and forces a lot of extra checks.