Fix watch issue on Windows

[bdukes]

When running elm-review --watch on Windows, the first review works successfully, but after a change is made to a file, the following error occurs:

-- ELM-REVIEW ERROR ----------------------------------------------- GLOBAL ERROR

: Found several modules named `Views.Example.Main`

I found several modules with the name `Views.Example.Main`. Depending on how
I choose to resolve this, I might give you different reports. Since this is a
compiler error anyway, I require this problem to be solved. Please fix this then
try running `elm-review` again.

Here are the paths to some of the files that share a module name:
- Website/DesktopModules/MVC/Example/Views/Example/Main.elm
- Website\DesktopModules\MVC\Example\Views\Example\Main.elm

It is possible that you requested me to look at several projects, and that
modules from each project share the same name. I don't recommend reviewing
several projects at the same time, as I can only handle one `elm.json`.

It appears that the first run uses file paths that have been made OS-agnostic (i.e. which uses / instead of \), but the path from the watcher is not made OS-agnostic, so it creates a situation where elm-review thinks the same file is at two different paths.

The initial loading of files in elm-files.js calls OsHelpers.makePathOsAgnostic on the path.

node-elm-review/lib/elm-files.js

Lines 158 to 163 in d35e821

	return {
	path: OsHelpers.makePathOsAgnostic(relativeFilePath),
	source,
	lastUpdatedTime,
	ast: cachedAst || (await readAst(options, elmParserPath, source))
	};

This function matches the removeWindowsSeparators function in watch.js, so I replaced its usages, as well as adding usages that were missing.

node-elm-review/lib/os-helpers.js

Lines 1 to 3 in d35e821

	function makePathOsAgnostic(path_) {
	return path_.replace(/.:/, '').replace(/\\/g, '/');
	}

node-elm-review/lib/watch.js

Lines 291 to 293 in d35e821

	function removeWindowsSeparators(string) {
	return string.replace(/.:/, '').replace(/\\/g, '/');
	}

[jfmengels]

Hey @bdukes!
Sorry for the delay in looking at your PR, I completely missed it!
I can't test this on Windows, but if it works for you then I'm good with it, since the changes look good.

Thank your for looking into this! ❤️