Support lark.indenter.Indenter in from_lark

[DRMacIver]

Lark has the general notion of a postlexer which allows you to do more complicated things with lexing than you can easily support in the main grammar definition language. Currently this will result in an error or, worse, an incorrect result.

A particular example of this that we need for #1806 is the Indenter class. Support for general lexers is hard (and may be impossible in the general case), but the indenter is well enough defined that we can just special case it.

[ThatXliner]

What's the status for this?

[Zac-HD]

Since nobody has either offered funding or volunteered to write the feature, it's been inactive.

We remain happy to accept PRs and open to grants or consulting of course, but it's a low priority for the volunteer maintainers and there's a lot more to do.