Only expand abbreviations if cursor is directly at command token

On a commandline like "ls arg" (cursor at end) we do not expand
abbrevations on enter.  OTOH, on "ls " we do expand. This can be
frustrating because it means that the two obvious ways to suppress
abbrevation expansion (C-Space or post-expansion C-Z) cannot be used to
suppress expansion of a command without arguments.  (One workaround is
"ls #".)

Only expand-on-execute if the cursor is at the command name (no space
in between).

This is a strict improvement for realistic scenarios, because if there
is a space, the user has already expressed the intent to not expand
the abbreviation. (I hope no one is using recursive abbreviations.)

Closes #8423

tests/checks/tmux-abbr.fish

@@ -0,0 +1,40 @@
+#RUN: %fish %s
+#REQUIRES: command -v tmux
+
+set -g isolated_tmux_fish_extra_args -C '
+    set -g fish_autosuggestion_enabled 0
+    function abbr-test
+    end
+    abbr -g abbr-test "abbr-test [expanded]"
+'
+isolated-tmux-start
+
+# Expand abbreviations on space.
+isolated-tmux send-keys abbr-test Space arg1 Enter
+tmux-sleep
+# CHECK: prompt {{\d+}}> abbr-test [expanded] arg1
+
+# Expand abbreviations at the cursor when executing.
+isolated-tmux send-keys abbr-test Enter
+tmux-sleep
+# CHECK: prompt {{\d+}}> abbr-test [expanded]
+
+# Use Control+Z right after abbreviation expansion, to keep going without expanding.
+isolated-tmux send-keys abbr-test Space C-z arg2 Enter
+tmux-sleep
+# CHECK: prompt {{\d+}}> abbr-test arg2
+
+# Or use Control+Space ("bind -k nul") to the same effect.
+isolated-tmux send-keys abbr-test C-Space arg3 Enter
+tmux-sleep
+# CHECK: prompt {{\d+}}> abbr-test arg3
+
+# Do not expand abbrevation if the cursor is not at the command, even if it's just white space.
+# This makes the behavior more consistent with the above two scenarios.
+isolated-tmux send-keys abbr-test C-Space Enter
+tmux-sleep
+# CHECK: prompt {{\d+}}> abbr-test
+
+# CHECK: prompt {{\d+}}>
+
+isolated-tmux capture-pane -p