build.py: updated path normalization for .js files [windows]

[sfinktah]

This is the one-line fix for the issue with building on Windows. The one that created ../../../../../../../../app.js paths.

-   rel_path = file.replace(f'{DIR_WEB}/', "")
+   rel_path = os.path.relpath(file, DIR_WEB)

This avoids the excessive changes in #609.

The declarations in comfy/frontend do still need to be made exportable, but there is probably a more appropriate place to deal with that.

[sfinktah]

@rgthree any change of expiditing this one? it's an absolute pain to have to patch manually for every PR.

... of course, you could probably do with less PRs. :)