relying on `last_*` statics was ugly and buggy.

previously if you cursored to the end of a long line (with softwrap
off), we'd set `viewport_x` and subsequently apply it even for short
lines narrower than buffer rect width. now we only apply `viewport_x`
when necessary.

commit 7231152c broke isearch highlighting because if an earlier rule
matched, the isearch rule would not. prepending the rule would not
work either as it would potentially prevent other srules from applying
normally. for example, if we had an srule for "foobar" and currently
isearching "foo", we'd either apply whatever srule appeared first --
the "foobar" srule or the isearch rule -- but not both. therefore,
similar to range_srules, we have to apply isearch rules in a separate
pass.

previously we'd look left for a quote, then right. if there was no
matching quote to the right, we gave up. the improved function will
continue at this point by looking right first, then left.

this is a rewrite of the syntax highlighting code. overall it's a
salmon-colored patch (deleted more than added), the code is easier to
follow, and it fixes some long-standing bugs.

as noted in the readme, there was a bug involving overlapping rules.
the most annoying instance of this bug was with strings containing a
`/*` such as in glob patterns. for example:

```
"match/this/path/*"
```

with `syn_generic` applied, we used to interpret `/*` as the
beginning of a multi-line comment, and therefore the rest of the file
(or until there happened to be a closing `*/` somewhere). this
bothered me for a very long time.

this bug was due to how we used to apply rules. each rule would get
applied one after the other, each from the beginning of the line. the
last rule to style any given span of text superseded any previous
rules that matched.

in the rewrite, once a rule matches a span of text, we don't try to
style that span anymore (until the text is edited of course).

the one downside is that this seems to be slightly less performant.
on my machine, applying `syn_generic` is about 5% slower, but it's
still generally fast enough that you'd only start to notice the
difference if you were opening massive files, tens or hundreds of
megabytes large, with syntax highlighting on. a memo field was added
(`smemo_t`) which drastically improved perf. there are probably
further perf optimizations to be had.

i added a couple more functional tests, on top of the existing ones,
to exercise some edge cases of syntax highlighting. as always please
report bugs if you find any.

not the most satisfying implementation but it does come in handy now
and then.

Mostly mandoc(1) WARNING and STYLE level issues found via `mandoc -Tlint`:

- new sentence, new line
- input text line longer than 80 bytes
- superfluous paragraph macro (Pp) before and after subsection macros (Ss)

as well as:

- sort options alphabetically
- capitalise Cygwin
- macOS starts with a lower case 'm'
- removed the sentence:

	Visit https://github.com/adsr/mle for more info.

  from the first paragraph as identical one is towards the end of
  the manual and that's where the URLs are usually found
- mark up hyperlink (Lk)
- typo: interpretted -> interpreted

instead of `chdir`ing the entire editor process, this patch maintains
a `browse_path` string that we `cd` into (as part of the forked shell
process) before executing `tree`. this should fix the bug
@kevinsjoberg reported without changing the behavior of `cmd_browse`.