fix(edit): require subr-x in klammertext-align.el (Emacs 27)

klammertext-align.el calls string-trim, string-blank-p, and
string-empty-p without requiring subr-x.  Newer Emacs has that library
loaded already; on Emacs 27 -- what Ubuntu 22.04 and Pop!_OS 22.04 ship
-- the symbols are void and table alignment fails with "Symbol's
function definition is void: string-blank-p".  The README now states
Emacs 27.1 as the supported floor.

(from dev 43a4ce7dea2a)

doc/edit/emacs/README.md

@@ -4,6 +4,9 @@
 helps Klammertext authors see the structure of klammer application through
 syntax highlighting.
 
+Requires **Emacs 27.1 or newer** — the version Ubuntu 22.04 and Pop!_OS
+22.04 ship, so it is the practical floor.  Tested on 27.1 and 30.2.
+
 ## Install
 
 Put the `emacs` directory somewhere on your system, then tell Emacs where it is