Add a new message NPPM_GETBUFFERIDFROMLEXER

[blu3mania]

Add a new message NPPM_GETBUFFERIDFROMLEXER for external lexer to query for buffer ID by providing lexer instance pointer. Enhanced Buffer to cache associated external lexer pointer, and FileManager to provide a lookup method to find the buffer in its list that is associated with the given lexer instance.

Without such a mechanism, it is impossible for lexer to know which instance should be used to handle messages received from Scintilla.

This closes #12351.