docs: recommend interface + symbol over class tokens for services

[sdirix]

What it does

This is a proposal to reverse the "classes over interfaces + symbols" coding guideline. For replaceable services (most public services in Theia), we should recommend declaring an interface + symbol with a default implementation, instead of using a concrete class as both the injection token and the type.

A class token forces adopters to subclass the default implementation. Splitting interface and symbol lets adopters choose: subclass the default, or provide a fully independent implementation. It also makes testing easier, since a test double can satisfy the interface directly.

An exception are internal services that are not meant to be extension points, where a plain class as token/type is fine.

How to test

Nothing to test

Follow-ups

• Align the codebase with the new recommendation

Breaking changes

• This PR introduces breaking changes and requires careful review. If yes, the breaking changes section in the changelog has been updated.

Attribution

Review checklist

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

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