refactor: consolidate and expand ls function family

- Upgrade ls to long listing (-l -a) and drop the standalone l and lS functions in favor of abbreviations
- Remove llm (ambiguous name, redundant with ltr)
- Add lsr (reversed time oneline), lss (size-sorted), lD (dirs-only), lx (extension-sorted)
- Enhance ltr with --all and age color-scale gradient
- Wire up ls abbreviations: l, lS, lsR, lX, lT, lsT
- Update README functions table and add Listing abbreviations section

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

functions/lx.fish

@@ -0,0 +1,12 @@
+# Copyright (C) 2026 Rootiest
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+function lx --description 'Extension-sorted listing'
+    if which eza >/dev/null 2>&1
+        eza --long --all --sort=extension --icons --color=auto --hyperlink $argv
+    else if which lsd >/dev/null 2>&1
+        lsd --long --all --sort=extension --hyperlink=auto $argv
+    else
+        command ls --color=auto -lX $argv
+    end
+end