Make External Lexer Plugin work with v8.4

[donho]

In Notepad++ v8.4, Scintilla is upgrade from v4.x to v5.x and Lexilla module has been added in the project. With such major update, the old External Lexer Plugins need to migrate to Scintilla5 and add new CreateLexer export function, which return a pointer of ILexer5 instance, in your external lexer plugin to make it work under v8.4.

Here’s the official info for updating your lexer to be compatible with Lexilla:
https://www.scintilla.org/LexillaDoc.html

An additional diff of migration done in an external lexer plugin Rainmeter may give you an idea for the migration of your external lexer:
https://github.com/poiru/rainlexer/pull/3/files