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/klammertext-align.el

@@ -50,6 +50,11 @@
 
 ;;; Code:
 
+;; string-trim, string-blank-p, and string-empty-p live in subr-x, which is
+;; not loaded by default in a batch Emacs before 28.  Without this require
+;; they are void on Emacs 27 (as shipped by Pop!_OS 22.04) while working on
+;; newer Emacs, so the alignment command fails on the older editor only.
+(require 'subr-x)
 (require 'klammertext-mode)
 
 (defgroup klammertext-align nil