REPL: switch between modes via a menu

[rfourquet]

The menus pops up via the , key.
Modes selected via , are sticky.
Help and shell modes remain non-sticky when entered via ; and ?.

Multiple keys have been proposed for accessing the menu, e.g. [ (in #14081 (comment)), but this is now taken, and , seems convenient.

This is implemented with TerminalMenus:

It was suggested that you can use another key after the initial , to quickly select your mode: this would be future work, as an improvement of TerminalMenus, which I believe does't provide the necessary feature.

[rfourquet]

Bump. Should this get the triage label?

[brenhinkeller]

So looks like this still needs docs and tests, but otherwise could potentially move forward and be discussed in triage?

Amazingly still no merge conflicts after three years!

[LilithHafner]

Triage likes this with a few caveats and ideas

• A major use could be for packages that add new repl modes (that way they don't have to coopt random keystrokes)
• For packages with 1-character keystroke triggers, list that trigger in the menu
• It seems a bit too complex for the stickyness of a mode to depend on how you got there (and there isn't all that much use for sticky help mode)
• This is feature-y and we don't want more features (but this will be the last repl-mode keystroke we ever add so it's okay)
• , is literally the only remaining non-shift character on a standard US keyboard that cannot start a line (aside from escape)

[sefffal]

Would escape (at the start of the line) be an option? It might be more discoverable, and the menu could even include an "exit" item. Those could really help beginners.