fix(link): split links with string.split captured at load

LinkUtil.SplitLinkData and SplitLinkOptions looked up string.split on
every call. BigWigs (pepopo978 fork) defines a table-returning
string:split in Plugins/SpellRequests.lua, which replaces the global, so
with it loaded every custom link type resolved to a table, no handler
matched, and SetItemRef rejected the link as unknown.

string.split is registered in C by the DLL before any addon loads, so a
file-local captured when LinkUtil.lua loads always holds the real one.

Fixes brues-code/pfUI#73

src/embed/AddOns/!!!ClassicAPI/Util/LinkUtil.lua

@@ -1,3 +1,5 @@
+local strsplit = string.split;
+
 LinkTypes = {
 	AADCOpenConfig = "aadcopenconfig",
 	Action = "action",
@@ -75,7 +77,7 @@ function LinkUtil.FormatLink(linkType, linkDisplayText, ...)
 end
 
 function LinkUtil.SplitLinkData(linkData)
-	local linkType, linkOptions = string.split(":", linkData, 2);
+	local linkType, linkOptions = strsplit(":", linkData, 2);
 	linkOptions = linkOptions or "";  -- Could be nil if there's no ":" in linkData.
 	return linkType, linkOptions;
 end
@@ -85,7 +87,7 @@ function LinkUtil.SplitLink(link) -- returns linkText and displayText
 end
 
 function LinkUtil.SplitLinkOptions(linkOptions)
-	return string.split(":", linkOptions);
+	return strsplit(":", linkOptions);
 end
 
 -- Extract the first link from the text given, ignoring leading and trailing characters.