normalizeSlashes should probably no-op on *nix

[amcasey]

As @dmichon-msft pointed out, Linux file names can contain backslashes, so indiscriminately converting them into forward slashes is probably a bad idea. Redefining normalizeSlashes as the identity function on non-Windows platforms would also be a perf win, since it's very hot code. The biggest concern is that someone might be depending on this (e.g. I think you can use backslashes in paths in tsconfig.json).

[dmichon-msft]

It would probably be best practice to warn or even error if any paths provided in config files or source code contain back slashes, with a note that they will have different behavior depending on OS.

[MartinJohns]

@dmichon-msft Related to that statement: #44053