New lexer / parser based on Lark

[derula]

A new lexer and parser are in development, They are currently much slower than the sievelib parser, but should also be more accurate / resistant to errors. Plus, since they are based on a grammar, they are more easily extendable / bugs should be easier to fix.

To do:

• Rewrite Scintilla lexer to use new lexer
• Run new parser to find errors
  • Make sure Parser only runs some time after typing stopped, because it is so slow
  • Consider finding all parse errors at once and adding a panel to show all of them => Add list of all parse errors #31
• Consider adding more features to the grammar, e.g. Sieve extensions => Add more grammar features #32

See also #16.