scriblib/figure: add `#:continue?' option
Fix problem handling `examples` and `interaction`.

Unlike plain `racketblock`, `examples` and `interaction` are
"nested". As a result we emitted bad Markdown like:

    ```racket
    Examples:
    ```racket
    some-code
    ```
    ```

Markdown code blocks can't nest, so this needs to be:

    ```racket
    Examples:
    some-code
    ```

Also: Updated the unit test with examples of `examples` and
`interaction`.
Combine adjacent code spans into one.

These result from something like

    @racket[(x y)]

being treated by Scribble as multiple RktXXX items rather than one. As
a result the Markdown emitted was:

    `(``x`` ``y``)`

But obviously instead we want:

    `(x y)`

Kludgosity alert: Although it would probably be more-correct to
consolidate the RktXXX items at the Scribble structure level, I don't
easily see how. `@racket` is baking in the concept of Racket
lexing (classifying text as various kinds of Racket elements). This is
handy when the render will be HTML or Latek, and is benignly N/A when
it will be plain text. But it's a bit square-peg/round-hole when the
render will be Markdown. Rather than attempt to "un-lex" the Scribble
structures (I'm having trouble seeing how), I'm handling it
"after-the-fact" -- adjusting the generated Markdown text.

If anyone thinks the preceding is an elaborate rationalization for an
ugly kludge, I wouldn't argue, but I would need some help
understanding the preferable way to go about it.
scriblib/footnote: fix for non-Latex output

Also, fix warnings generated even for Latex mode.
reference: fix for reader's number grammar
syntax-color/racket-lexer: fix long-standing number-parsing bug

The lexer mishandled cases like "#x1E+2", because "E" is
not an exponent marker for hexadecimal --- and it has been that
way forever, but the repair isn't all that difficult.
syntax-color/racket-lexer: update for single-precision and extflonums

Includes a doc fix to note `+inf.t', etc.
reference: doc clarifications on `guard-evt'
contract error: shift path normalization to error time

Making a pathcollects-relative  at contract-formation time means a
lot of extra work then and a change to the needed sandbox permissions.
Aormalizing at error time is more consistent with other parts of a
contract message.
doc repair

Closes PR 13555