Codechange: [Script] Use helper function over direct casting CompanyID

In the script's API `COMPANY_INVALID` has a value of -1, whereas the internal
game's `INVALID_COMPANY` has a value of 255. Since the script's API also has
a `COMPANY_SPECTATOR` with a value of 255, these enumerations cannot be easily
reconciled by casting. As such, replace all casts in the script API with
either ScriptCompany::FromScriptCompanyID or ScriptCompany::ToScriptCompanyID.

Also make clear whether CompanyID is ::CompanyID or ScriptCompany::CompanyID
by using either one of those over CompanyID in the script's API.

src/script/api/script_grouplist.cpp

@@ -17,7 +17,7 @@
 ScriptGroupList::ScriptGroupList(HSQUIRRELVM vm)
 {
 	EnforceCompanyModeValid_Void();
-	CompanyID owner = ScriptObject::GetCompany();
+	::CompanyID owner = ScriptObject::GetCompany();
 	ScriptList::FillList<Group>(vm, this,
 		[owner](const Group *g) { return g->owner == owner; }
 	);