Add dedicated RStudio keymap

[jmcphers]

This change adds an RStudio keymap to Positron, and implements all of the keybindings that have been specifically requested by alpha users. Namely:

• Ctrl + 1 to focus Source
• Ctrl + 2 to focus Console
• Cmd + . to go to symbol
• Cmd + Shift + C to comment/uncomment a line
• Cmd + Shift + N to create a new R file
• F2 to go to definition
• Cmd + I to reindent selection
• Cmd + Shift + A to reformat selection
• Alt + Shift + I to insert a new Quarto/R Markdown cell
• Cmd + D to delete the current line

There are a lot more keybindings we'll probably end up adding, but these seem to be causing the most friction.

Note that most of the bindings mentioned in #669 are already implemented in the R extension; we felt it was okay to take most of these bindings when the user is editing an R package.

The keymaps are implemented in a new extension which is currently codeless, but will probably have code later so we can add custom RStudio-like logic for other keys/commands. As there is no way to see or disable built-in or bundled extensions, we gate all the extension's bindings on a setting that can be toggled here:

Turning on this setting activates the keybindings (no restart necessary!).

Addresses #669.

[jmcphers]

As there is no way to see or disable built-in or bundled extensions, we gate all the extension's bindings on a setting that can be toggled

Another way to do this might be to have the extension activate on a command. That would be a little more complicated to implement (a setting in a different extension might have to run the command? dynamically loading it at startup when enabled might be harder?), but it would make things cleaner from a user perspective -- right now you always see the RStudio Keymap items in the Keyboard Shortcuts dialog (with the config flag); if we enabled on a command then you'd only see them if the keymap was actually turned on.

[jmcphers]

Another way to do this might be to have the extension activate on a command

I just tried this (e1552bc) and it doesn't work the way I thought. It appears based on my testing that the keybindings contributions are active even if the extension isn't activated. So just installing the extension is enough to turn on the keybindings, and since this extension is always installed, they're always active.

[softwarenerd]

LG!