System.FilePath.parent did not return the correct parent path in the
case of absolute file paths
Example of previous behavior
```
(FilePath.mk "/foo").parent -> some (FilePath.mk "")
(System.FilePath.mk "/").parent -> some (FilePath.mk "")
```
The new behavior is based on rust's std::path::Path::parent function (as
previously described in comment in System.FilePath)
Example of updated behavior
```
(System.FilePath.mk "/foo").parent -> some (FilePath.mk "/")
(System.FilePath.mk "/").parent -> none
```
Behavior for relative file paths is unchanged
Closes#3618
This introduces `FilePath.addExtension` to take a path that we know has
no prior extension, and append a new extension to it.
As this function is simpler than `FilePath.withExtension`, this change
eagerly replaces uses of the latter with the former, except in a few
cases where stripping the extension really is the right thing to do.
This should fix the bug described at
https://leanprover.zulipchat.com/#narrow/stream/270676-lean4/topic/Import.20file.20with.20multiple.20dots.20in.20file.20name/near/404508048,
where `import «A.B».«C.D.lean»` is needed to import `A.B/C.D.lean`.
Closes#2999
---------
Co-authored-by: Mac Malone <tydeu@hatpress.net>
Co-authored-by: Sebastian Ullrich <sebasti@nullri.ch>
