Use 'regexp-opt' in 'dired-omit-regexp'

Use 'regexp-opt' in 'dired-omit-regexp'

In my benchmarking, for large dired buffers, using 'regexp-opt'
provides around a 3x speedup in omitting.

'regexp-opt' takes around 5 milliseconds, so to avoid slowing
down omitting in small dired buffers we cache the return value.

Since omitting is now 3x faster, increase 'dired-omit-size-limit'
by 3x.  Also, document 'dired-omit-size-limit' better.

* doc/misc/dired-x.texi (Omitting Variables): Document
'dired-omit-size-limit'.
* etc/NEWS: Announce increase of 'dired-omit-size-limit'.
* lisp/dired-x.el (dired-omit--extension-regexp-cache): Add.
(dired-omit-regexp): Use 'regexp-opt'.  (Bug#69775)
(dired-omit-size-limit): Increase and improve docs.