Fix incorrect URI conversions in custom-editors-main.ts

[pisv]

What it does

Currently, custom-editors-main.ts contains multiple instances of Theia to VSCode URI conversions of the form URI.file(uri.path.toString()), which seem to be incorrect in the general case. This PR replaces them with URI.from(uri.toComponents()) or just uri.toComponents() as appropriate.

How to test

I don't have a specific test case for this, but hopefully it can be verified by code inspection.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[msujew]

Looks good to me. I can confirm that custom editors still behave as expected wrt. editing, saving and reloading.

[pisv]

@msujew Thank you very much for reviewing!