[std] Create new top-level Clause [text], following [strings]

Part of the C++26 clause restructuring (#5315).

Author: tkoeppe

source/std.tex

@@ -134,6 +134,7 @@
 \include{ranges}
 \include{algorithms}
 \include{strings}
+\include{text}
 \include{numerics}
 \include{time}
 \include{locales}

source/text.tex

@@ -0,0 +1,3 @@
+%!TEX root = std.tex
+
+\rSec0[text]{Text processing library}