Add virtual selection to integrated terminal

[Tyriar]

This will enable things like:

• Double clicking on the prompt line being flaky due to focus shifting from .xterm to the text area.
• Double clicking on a URL/path to highlight the whole path (ie. don't use / and similar chars as separators)
• Selecting and copying multiple pages of the buffer

Upstream issue: xtermjs/xterm.js#207

[Tyriar]

I'll treat this as the master issue for all related bugs, here's probably incomplete list of things this fixes:

• Double clicking on the prompt line being flaky due to focus shifting from .xterm to the text area
• Double clicking on a URL/path to highlight the whole path (ie. don't use / and similar chars as separators)
• Selecting and copying multiple pages of the buffer Selection is only possible on the text in the current viewport xtermjs/xterm.js#69
• Selection goes crazy when you start dragging and then use mouse wheel Selection goes crazy when you start dragging and then use mouse wheel xtermjs/xterm.js#68
• Making the selection's line height exact so there are never any gaps between lines Terminal rendering shows space between lines with zoomlevel:1 #18588
• Don't drop the selection when there's a content change or viewport scroll text-selection is lost when data is written outside of the viewport xtermjs/xterm.js#468 Spinners in console applications make selection difficult xtermjs/xterm.js#318
• Allow selection to be retained in terminal when not focused (like editor) 'Terminal: Copy Selection' no way to be executed from the command palette #9761
• Add select all command Select all does not work xtermjs/xterm.js#307 "Select all" not available in terminal #15199
• Selection is dropped sometimes when right clicking outside of selection Right clicking on a portion of the terminal that is not selected sometimes drops selection #23866
• Black marker is shown when context menu is closed A black marker is shown when context menu is closed #9758
• 'Terminal: Copy Selection' no way to be executed from the command palette 'Terminal: Copy Selection' no way to be executed from the command palette #9761
• Cannot select link in terminal Cannot select link in terminal #25155

And potentially fixes/impacts:

• Selection should not be visible during mouse interaction mode Selection should not be visible during mouse interaction mode xtermjs/xterm.js#140
• Improve tmux support Improve tmux support xtermjs/xterm.js#456
• Tmux ignores mouse scroll events/shits itself on selection events Tmux ignores mouse scroll events/shits itself on selection events xtermjs/xterm.js#329
• Copy/paste does not work in Edge Copy/paste does not work in Edge xtermjs/xterm.js#401
• Line height affects IME composition view wrong in terminal Line height affects IME composition view wrong in terminal  #13230