fix: clipping issue in debug hover (#15086)

[xpomul]

What it does

Fixes #15086 by adding a z-index property to the theia-debug-hover CSS class, as proposed in that issue.
The root cause was related to PR #14695 which has addressed overlay issues in the dock panels.

Notes:

1. I did a quick check of how z-index is used in Theia's css files but there seems no greater logic and z-indexes seem to be assigned on a "whatever works" basis. I chose 1000 because the Debug Hover should be on top of other things, but maybe not the Notification Panel (which has a z-index of 1111).
2. When applying the change and saving, the workspace formatter settings introduce several format changes. Obviously the affected CSS file is not in a proper format according to the Theia guidelines. I chose to not include the formatting changes into this PR to keep it small and simple.

How to test

Simplest way of testing:

• In the Debug Panel, check the "Caught Exceptions" breakpoint
• Launch "Start Browser Backend" and wait for the process to suspend
• Hover in the source editor (possibly resize the window in a way that the hover overlaps with docked panels)

See #15086 for a screenshot.

Follow-ups

See notes in the "What it does" section. The file packages/debug/src/browser/style/index.css should be reformatted according to the Theia project settings. (But most likely this is not the only file that is affected...?)

Review checklist

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