add "exists_as_package" script function

doc/function/exists_as_package.txt

@@ -0,0 +1,16 @@
+Function: exists_as_package
+
+--Usage--
+> exists_as_package("package")
+
+Check if a package exists in the data folder.
+
+Returns the package's version number as major-minor-patch, or nil if the package does not exists.
+
+--Parameters--
+! Parameter	Type			Description
+| @input@	[[type:string]]		Full name of the package folder, including game and extension.
+
+--Examples--
+> exists_as_package("magic-modules.mse-include")  ==  "2026-02-28"
+> exists_as_package("magic-quadruple-faced-leveler.mse-style")  ==  nil

doc/function/index.txt

@@ -138,4 +138,5 @@ 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:exists_as_package]]			Checks if a package exists.
 | [[fun:exists_in_package]]			Checks if a file exists in a package.