Move SSH feature to own package

[msujew]

What it does

Closes #15670

Splits the SSH feature from the @theia/remote package into its own @theia/remote-ssh package. Does so for better separate of concers and to allow adopters to add remote features without automatically adding the SSH features into the application.

How to test

Confirm that the Remote SSH feature still works as expected. Removing @theia/remote-ssh from the electron example app package should still allow to use the other remote (WSL, Dev-Container) features, while removing only the SSH related features.

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

[jonah-iden]

@msujew i think something might still be broken. At least for me it does not call the establishConnection method on backend side when i try to connect to host. So after the quick input nothing happens

[ndoschek]

Hi @msujew, this PR has been quiet for a while, just wondering if you are still planning to continue with that? TIA!

[msujew]

Jonah is currently on vacation. I'll update and look into the SSH issue once he's back :)