Dir: Add executablePath and workingDirectory

[bryphe]

In discussing the Fp.absolute* family of APIs in #251, we were discussing how hopefully users don't have to call these explicitly too often!

There are two more paths that would be useful to include in Dir that would commonly be used:

• Dir.executablePath(): unit => Fp.t(absolute) - the absolute path to the running executable. Probably want an option to explicitly say if symlinks should be resolved or not. Having a nice API for this could really streamline the various corner cases we need to handle in Revery, today: https://github.com/revery-ui/revery/blob/4173462fc4b53fc70761f35babf1a8b9767c5b4c/src/Core/Environment.re#L59

• Dir.workingDirectory(): unit => Fp.t(absolute) - the absolute path to the current working directory - a thin wrapper over Sys.getcwd() + Fp.absolute.