ui/ux: styling vsx and source control input fields

[vladarama]

What it does

Closes: #10855
This PR provides better styling for various input fields when the view is limited. This includes the vsx-registry and the source control views. These changes allow the placeholder overflow to be hidden and replaced by an ellipsis to make the styling cleaner and more consistent.

Demo:

How to test

• Start Theia, open the Extensions or the Source Control views
• Resize the view, making it smaller
• The input field should be responsive, with the text overflow hidden by an ellipsis

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

[vince-fugnitto]

LGTM 👍
The changes work well for me, and I can confirm the input fields are more responsive.

[vince-fugnitto]

@vladarama when you have the time do you mind rebasing to resolve the conflicts, and I'll merge.