Right arrow autocompletes at line end (#54983)

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.

Author: AshlinHarris

stdlib/REPL/src/LineEdit.jl

@@ -741,7 +741,26 @@ function edit_move_right(buf::IOBuffer)
     end
     return false
 end
-edit_move_right(s::PromptState) = edit_move_right(s.input_buffer) ? refresh_line(s) : false
+function edit_move_right(m::MIState)
+    s = state(m)
+    buf = s.input_buffer
+    if edit_move_right(s.input_buffer)
+        refresh_line(s)
+        return true
+    else
+        completions, partial, should_complete = complete_line(s.p.complete, s, m.active_module)
+        if should_complete && eof(buf) && length(completions) == 1 && length(partial) > 1
+            # Replace word by completion
+            prev_pos = position(s)
+            push_undo(s)
+            edit_splice!(s, (prev_pos - sizeof(partial)) => prev_pos, completions[1])
+            refresh_line(state(s))
+            return true
+        else
+            return false
+        end
+    end
+end
 
 function edit_move_word_right(s::PromptState)
     if !eof(s.input_buffer)