fix #13222: make relativePath more robust and flexible (#13451)

* * relativePath(foo) now works
* relativePath(rel, abs) and relativePath(abs, rel) now work (fixes #13222)
* relativePath, absolutePath, getCurrentDir now available in more targets (eg: vm, nodejs etc)
* fix bug: isAbsolutePath now works with -d:js; add tests
* workaround https://github.com/nim-lang/Nim/issues/13469
* remove `relativePath(path)` overload for now
* add back changelog after rebase

tests/stdlib/tos.nim

@@ -348,6 +348,9 @@ block ospaths:
   doAssert relativePath("", "foo") == ""
   doAssert relativePath("././/foo", "foo//./") == "."
 
+  doAssert relativePath(getCurrentDir() / "bar", "foo") == "../bar".unixToNativePath
+  doAssert relativePath("bar", getCurrentDir() / "foo") == "../bar".unixToNativePath
+
   when doslikeFileSystem:
     doAssert relativePath(r"c:\foo.nim", r"C:\") == r"foo.nim"
     doAssert relativePath(r"c:\foo\bar\baz.nim", r"c:\foo") == r"bar\baz.nim"