[WIP] Extended support for highlighting

[SuzanneSoy]

This patch:

• Adds support for custom CSS styles for (code:hilite code style-name)
• Fixes rendering of @racket[(a . (b c))] so that the dot is indeed shown if it were present in the source
• I tentatively added a case when determining the first element [(code:line . rest) #'rest]. I'm not sure it's correct, though.
• Adds support for code:hilite in a dotted tail position: (foo bar code:hilite baz) renders as (foo bar . baz), with the baz highlighted. (foo bar code:hilite baz style-name) should also work, and take into account the style name.
• Adds support for dotted tail elements to code:line, so that (code:line a b . c) renders as a b . c, just as (code:line a b c) renders as a b c.
• Fixes the error message given when a code:hilite is misused (previously: "bad code:redex: ~.s", now: "bad code:hilite: ~.s").
• Disabled the spurious (set! src-col (add1 src-col)) for code:hilite when the syntax-span of the whole form is 0. This allows tools to wrap elements with code:hilite forms, without having an extra space. The tool needs to give the whole (code:hilite e) form the same srcloc as e, except for the syntax-span which is overridden with 0.

These features / changes allow tools to highlight parts of the code, for example sexp-diff could be used to generate pretty diffs (such a tool would need to take care of srclocs, though), or to show before/after steps in literate programming. I wrote a small utility which does this:

The bulk of the diff comes from moving lloop out of its former cond case, so that it can be used both by the [(or (pair? (syntax-e c)) … case and by the code:line case.

Issues to solve before merging:

• I'm not sure if the [(code:line . rest) #'rest] is correct (first line of the diff). I think it won't cause any harm, but I'd like one of the scribble devs to double-check it.
• Documentation
• It would be nice to be able to specify a highlight style, instead of specifying a style name. Currently, it is necessary to manually inject the css-additions outside of the code. I'm not sure how to embed a style without resorting to 3D syntax, or performing a dynamic-require of some sort. Ideas welcome!

[bennn]

I think this case only matches (code:line), so it shouldn't hurt but also shouldn't change how anything looks.

[SuzanneSoy]

With the patch which I added, you can now write (code:line . foo) which renders as . foo (but previously it was not possible).

Also, technically, we could have a first element which is (code:line (code:line something)), should the code above be called recursively as long as code:line is the first element? (I can amend the patch to include that, if you think it's the right thing to do).

[SuzanneSoy]

Note to self: The problem when making sure that @racket[(a . (b c))] gets rendered as (a . (b c)) instead of (a b c), i.e. the second point of the PR, is that scribble's @defform produces nested syntax pairs, instead of producing a syntax list.

I would therefore either have to "fix" defform (and test the other forms, @defproc works fine but others may exhibit similar problems), or remove this part of the patch, or make it an option, e.g. accessible via @racket/dots[(a . (b c))].