Allow user control on what starts and ends a paragraph for bidi

Allow user control on what starts and ends a paragraph for bidi

* src/buffer.h (struct buffer): New members
bidi_paragraph_separate_re_ and bidi_paragraph_start_re_.
* src/buffer.c (bset_bidi_paragraph_start_re)
(bset_bidi_paragraph_separate_re): New setters/
(Fbuffer_swap_text): Swap the values of bidi-paragraph-start-re and
bidi-paragraph-separate-re.
(init_buffer_once): Init the values of bidi-paragraph-start-re and
bidi-paragraph-separate-re.
(syms_of_buffer) <bidi-paragraph-start-re, bidi-paragraph-separate-re>:
New per-buffer variables.
* src/bidi.c (bidi_at_paragraph_end, bidi_find_paragraph_start):
Support bidi-paragraph-start-re and bidi-paragraph-separate-re.
(bidi_move_to_visually_next): Handle correctly the case when the
separator matches an empty string.  (Bug#27526)

* doc/emacs/mule.texi (Bidirectional Editing):
* doc/lispref/display.texi (Bidirectional Display): Document
bidi-paragraph-start-re and bidi-paragraph-separate-re.

* etc/NEWS: Mention bidi-paragraph-start-re and
bidi-paragraph-separate-re.