This adds helper functions to determine the first or last directory separator
in a string and to check if a given character is a separator.
Paths starting with a separator are now also considered absolute on
Windows as these are rooted at the current drive.
Note that it's fine to use DIRECTORY_SEPARATOR when combining strings as
Windows API calls accept both forward and backward slashes as separators.
Co-authored-by: Michał Skalski <mskalski@enigma.com.pl>
References #3684.
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