Right arrow autocompletes at line end

[AshlinHarris]

This commit adds a REPL feature: autocompletion when the right arrow key is pressed at the end of a line (The current behavior is to do nothing).

This new effect only occurs when:

• the cursor is at the end of the buffer,
• there is only one possible completion, and
• more than 1 character is in the buffer.

In this situation, the right arrow key behaves like <Tab>. Otherwise, the right arrow key behaves as normal.

The feature was requested in #54539 and seems intuitive to me. One useful side effect is that, by requiring that the cursor be at the end of the line, it offers a way to avoid autcompletes within words, which I assume are almost never helpful (For example, if I type "show" in the REPL, move the cursor onto the 'o' and press <Tab>, I end up with "showow"). One potential drawback is that the autocomplete could occur when a user who simply wants to move the cursor to the end of the line holds down the right arrow.

Resolves #54539

[fingolfin]

Looks good to me. I am not sure whether I'd use it in practice; or whether it even might annoy me, by triggering accidentally when I want to move to the end of the line -- though I usually use a hotkey like ctrl-e to move to the end of the line, so I am not worried. But others might find it annoying... Well, best way to find out from POV is to merge it and see what happens, and whether anyone complains. We can always revert it or put it behind a feature flag.