Increase `so-long-threshold' and `so-long-max-lines' defaults

Increase `so-long-threshold' and `so-long-max-lines' defaults

* lisp/so-long.el (so-long-threshold, so-long-max-lines): Increase
default values to reduce false-positives.

* etc/NEWS: Describe changes.

Lines shorter than 10,000 characters shouldn't generally be causing
problems, so testing this explicitly will largely eliminate
false-positives.  We must also increase the maximum number of lines
to check, because 'minified' code may still include newlines, and so
there may be many lines shorter than the new threshold before we find
a line which exceeds it.

Previously we used a minimum-effort heuristic, testing a very small
number of lines against a maximum length which, while not remotely
long enough to cause problems, would nevertheless be uncommon in any
normal file of programming code (and hence indicative that the file
was likely to be minified code).

Testing indicates that the performance penalty for the larger values
should be negligible.