[WIP] Implement text selection

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original issue description:

To help with text selection with the trackpad we will add a new text selection mode to hterm:

• when trackpad is clicked one time (corresponds to left mouse button click) the terminal enters text selection mode
• as the trackpad moves, the cursor selects text left/right and up/down
• when trackpad is clicked while in text selection mode then hterm leaves text selection mode and the text remains selected

You do not need to worry about copy/paste, there will be a separate menu to trigger copy and paste actions once the text is selected.

Remember to update the README with this new functionality.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Implement text selection</issue_title>
<issue_description>To help with text selection with the trackpad we will add a new text selection mode to hterm:

• when trackpad is clicked one time (corresponds to left mouse button click) the terminal enters text selection mode
• as the trackpad moves, the cursor selects text left/right and up/down
• when trackpad is clicked while in text selection mode then hterm leaves text selection mode and the text remains selected

You do not need to worry about copy/paste, there will be a separate menu to trigger copy and paste actions once the text is selected.

Remember to update the README with this new functionality.</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #3

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