Prevent access violations in older editors

The transition to Scintilla 4.4.6 brought with it a safer external lexer
implementation, patched in as an out-of-tree module [^1]; but N++
versions < 7.9.4 unconditionally follow a function pointer to
'GetLexerFactory', which was left out by 99b134c

 ----
 [^1] https://github.com/notepad-plus-plus/notepad-plus-plus/blob/d7819cf601051510b7fa1640ca17fedad297e1f8/scintilla/src/ExternalLexer.cxx#L131-L135

Author: rdipardo

Source/Plugin/NPPFSIPlugin.lpr

@@ -236,6 +236,7 @@ procedure DLLEntry(dwReason: DWORD);
 {$ENDREGION}
 exports
   GetLexerCount,
+  GetLexerFactory,
   GetLexerName,
   GetLexerStatusText,
   CreateLexer,

Source/Plugin/Src/ILexerExports.pas

@@ -15,6 +15,7 @@ function CreateLexer(const Name: PAnsiChar): NativeInt; stdcall;
 procedure GetLexerName({%H-}LexerIndex: Cardinal; Name: PAnsiChar; BufLength: Integer); stdcall;
 
 /// Unused since Npp v8.4, but needed to prevent a load exception in older versions
+function GetLexerFactory({%H-}LexerIndex: Cardinal): NativeInt; stdcall;
 procedure GetLexerStatusText({%H-}LexerIndex: Cardinal; Name: PWideChar; BufLength: Integer); stdcall;
 
 implementation
@@ -40,6 +41,11 @@ procedure GetLexerName({%H-}LexerIndex: Cardinal; Name: PAnsiChar; BufLength: In
   StrLCopy(Name, PAnsiChar(lexerName), BufLength);
 end;
 
+function GetLexerFactory({%H-}LexerIndex: Cardinal): NativeInt; stdcall;
+begin
+  Result := 0;
+end;
+
 procedure GetLexerStatusText({%H-}LexerIndex: Cardinal; Name: PWideChar; BufLength: Integer); stdcall;
 const
   lexerName: WideString = 'F# source file';