Download scintilla and look into scintilla.h. There you will find all scintilla commands and constants with their values.
If you are going to use scilexer.dll then you don't need .DECLS file for that DLL. Instead, you'll need some kind of constants definition file, because all communication with Scintilla is done through messages, i.e. one function that you use to send command and parameter values. You won't find any scintilla functions exposed in SciLexer.dll .
However, based on those Constants, you can create your own functions, and based on that, library with Scintilla functions and .Decls file for that library.
If you are going to use scilexer.dll then you don't need .DECLS file for that DLL. Instead, you'll need some kind of constants definition file, because all communication with Scintilla is done through messages, i.e. one function that you use to send command and parameter values. You won't find any scintilla functions exposed in SciLexer.dll .
However, based on those Constants, you can create your own functions, and based on that, library with Scintilla functions and .Decls file for that library.