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.