add "version_is_older" script function

doc/function/index.txt

@@ -138,5 +138,6 @@ These functions are built into the program, other [[type:function]]s can be defi
 | [[fun:assert]]		Check a condition for debugging purposes.
 | [[fun:warning]]		Output a warning message.
 | [[fun:error]]			Output an error message.
+| [[fun:version_is_older]]			Checks if a version number is older than another.
 | [[fun:exists_as_package]]			Checks if a package exists.
 | [[fun:exists_in_package]]			Checks if a file exists in a package.

doc/function/version_is_older.txt

@@ -0,0 +1,18 @@
+Function: version_is_older
+
+--Usage--
+> version_is_older("version", than: "other version")
+
+Check if a version number is strictly older than another.
+
+Malformed version numbers, including nil, are treated as 0-0-0.
+
+--Parameters--
+! Parameter	Type			Description
+| @input@	[[type:string]]		Version we want to check.
+| @than@	[[type:string]]		Version we want to check against.
+
+--Examples--
+> version_is_older("2025-06-28", than: "2026-02-18")  ==  true
+> version_is_older("2026-02-18", than: "2026-02-18")  ==  false
+> version_is_older(nil, than: "0-0-1")  ==  true