Add isPathAbsolute() function and remove inconsistency in naming

[IndrajeetPatil]

All other validators follow the same pattern:

• the underlying check is implemented in function is*() or has*() function
• the validation is implemented in function validate*() function

Currently, validatePathIsAbsolute() is the only exception that doesn't have a corresponding is*() function.

Also, this function is inconsistent with the naming convention. The names should be:

• isPathAbsolute()
• validateIsPathAbsolute()

Maybe this can be rectified by using an alias. That way, this won't be a breaking change.