Replace `lexer` getter and setter functions with indexable properties.
This follows Textadept's Scintilla API.
Make end delimiters optional in `lexer.nested_pairs()`.
After so many years, this just makes sense now.
Change `lexer.delimited_range()` API.
In the beginning these options made sense, but after many years, use some sane
assumptions to simplify the API.
Updated themes to use new lexer property tables.
Default to fold by indentation in whitespace-significant languages.