Improve Inline Tip in VSCode

[Link4565]

Description

Remove background and outline of tip from SVG. This is required since the SVG content of a decoration cannot be taller than the line text height without causing the entire line to be pushed down. Fixes #4324

Replace the box outline and background with CSS.

Fix spacing of text inside box. Add '+' symbols to shortcuts to make them clearer.

Checklist

• The relevant docs, if any, have been updated or created
• The relevant tests, if any, have been updated or created

Screenshots

Before:

After:

Testing instructions

1. Install extension in Visual Studio Code
2. Open a non-empty file
3. Select some text
4. Verify that the updated inline tip opens, and that the other text content on the line does not move down.

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[sestinj]

this looks great!