Add external formatter support

[6cdh]

This PR switches the existing handwritten formatting logic to external formatters.

Currently, only racket-fixw is integrated because it's error-tolerant, and supports interactive mode.

• For on type formatting, use interactive mode. It indents for empty lines as if there is an atom on these lines based on the context. So if user types enter, they are at the indented position, not the beginning of the line.
• For full document and range formatting, use normal mode, that does not indent for empty lines because they are treated as trailing spaces.

Resolve #178

And after this change, we can use a handwritten text buffer to replace racket:text% class, and remove the GUI framework dependency.