Feat preview color scheme on mouse hover

[eleadufresne]

Summary of the Pull Request

Allow users to preview color schemes by hovering over them with the mouse pointer in the Command Palette.

References and Relevant Issues

• This PR handles issue Request: "Select color scheme..." mouse-hover should provide color-scheme preview #18238.
• This extends the previously closed issue Allow commands to be previewed in the command palette #6689, which allowed the UP and DOWN arrows to trigger a preview of color schemes in the Command Palette.

Detailed Description of the Pull Request / Additional comments

This works by attaching event handlers for PointerEntered and PointerExited to ListViewItem containers. When the mouse pointer moves into the item's bounding area, the PreviewAction handler is triggered to showcase the hovered color scheme. Conversely, when the mouse pointer leaves the item's area, the PreviewAction is executed on the selected item (generally from the UP and DOWN arrows).

Decisions I made, but I'm uncertain about:

• I coded my solution so that PreviewAction is called on the submenu items, as this is where the color schemes are located. I felt invoking the function for each element was unnecessary, but this approach may make it less flexible for future modifications.
• I'm attaching pointer event handlers when the data associated with the ListViewItem changes, but there might be a better approach.

Important note:

• This also provides previews for the other features that the ActionPreviewHandler handles, such as the background opacity of the terminal.

Validation Steps Performed

• Hover a color scheme, and it becomes the active one.
• Pressing ESC at any point to dismiss the command palette, and the scheme returns to the previous one.
• I did not add any additional test, though all existing ColorScheme tests passed.

PR Checklist

• Closes Request: "Select color scheme..." mouse-hover should provide color-scheme preview #18238
• Tests added/passed
• Documentation updated
  • If checked, please file a pull request on our docs repo and link it here: #xxx
• Schema updated (if necessary)

[eleadufresne]

@microsoft-github-policy-service agree

[carlos-zamora]

Looks and feels great! Just the one comment before approving. Thanks for doing this!

[carlos-zamora]

(whoops, clicked the wrong button...)