[fish] exit as well when called from non-interactive shell (#3467)

Just like with the other shells, exit fish to, if called from a non-interactive
shell.

We cannot use `return`, as older versions of fish (namely < 3.4.0) did not
support to use `return` in `.`-scripts (this was only added with fish commit
3359e5d2e9bcbf19d1652636c8e448a6889302ae).

Unlike in POSIX, fish’s `exit` is however documented to no cause the calling
shell to exit when executed in a sourced script (see:
0f70b2c0d3/doc_src/cmds/exit.rst (L20)
)

Signed-off-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>
Co-authored-by: Junegunn Choi <junegunn.c@gmail.com>

shell/key-bindings.fish

@@ -11,6 +11,9 @@
 # - $FZF_ALT_C_COMMAND
 # - $FZF_ALT_C_OPTS
 
+status is-interactive; or exit 0
+
+
 # Key bindings
 # ------------
 function fzf_key_bindings