Consider TerminalLinkProvider contributions recursively

[xpomul]

What it does

Fixes #15050 by considering the TerminalLinkProvider contributions also recursively (i.e., consider TerminalLinkProvider contributions bound in the root container).

For a thorough analysis of the issue see #15050 (comment)

How to test

See #15050 description.
In short:

• npm run build:browser
• npm run download:plugins
• npm run start:browser
• Start frontend and make sure a folder is opened
• Command Palette: Terminal: Create new Terminal
• In the terminal, execute ls -l
• Try to hover over a file (e.g., package.json)

Without the fix, there is no link provided, with the fix, the link is there and clicking leads to the file being opened in an editor.

Notes:

1. I am not 100% sure whether this is the correct fix. It could also be a bug that plugin-ext binds the TerminalServiceMainImpl in the child container, not the parent (root) container. So, maybe someone with knowledge in this area should have a second look. (Again, see the linked comment in the issue for details)
2. I tried to write a unit test but failed because importing the terminal-link-provider from the test results in a phosphor error (ReferenceError: navigator is not defined). So I am not sure how to mock the UI dependencies correctly and I haven't found a similar example/test case in the code base...

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