## PR Checklist
* [x] Closes https://github.com/microsoft/terminal/issues/9725
* [x] CLA signed.
* [ ] Tests added/passed
* [ ] Documentation updated.
* [ ] Schema updated.
* [ ] I've discussed this with core contributors already.
## Validation Steps Performed
* [x] single click = no selection
* [x] single click and drag = selection starting from first point
* [x] single click in unfocused pane and drag = focus pane, selection starting from first point
* [x] double-click = selects a whole word
* [x] triple-click = selects a whole line
* [x] double-click and drag = selects a whole word, drag selects whole words
* [x] triple-click and drag = selects a whole line, drag selects whole lines
* [x] Shift single-click = defines start point
* [x] second Shift single-click = defines end point
* [x] Shift double-click = selects entire word
* [x] Shift triple-click = selects entire line
* [x] Shift double-click and drag = selects entire word, drag selects whole words
* [x] Mouse mode: Shift single-click = defines start point
* [x] Mouse mode: second Shift single-click = defines end point
* [x] Mouse mode: Shift double-click = selects entire word
* [x] Mouse mode: Shift triple-click = selects entire line
* [x] Mouse mode: Shift double-click and drag = selects entire word, drag selects whole words
* [x] With existing selection: single-click outside the selection and drag = establishes a new selection starting from the click point
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