vim-patch:8.2.{0877, 0880, 0884, 0887, 0896} - port searchcount()

Problem:    Line() only works for the current window.
Solution:   Add an optional argument for the window to use.
https://github.com/vim/vim/commit/8e0a8e7eb7c177807f44db6b76d8e52314248ab5

Problem:    Cannot get the search statistics.
Solution:   Add the searchcount() function. (Fujiwara Takuya, closes vim/vim#4446)
https://github.com/vim/vim/commit/e8f5ec0d30b629d7166f0ad03434065d8bc822df

Additional changes:
- Tests weren't passing because the test ran assuming the cursor was at
  start of buffer but append() left the cursor at end of buffer .
  So cursor is moved to start of buffer after append.
- searchcount() added to list of builtin functions.

Problem:    Win_execute() is not implemented yet.
Solution:   Implement it.
https://github.com/vim/vim/commit/868b7b6712ea4f2232eeeae18c5cbbbddf2ee84d

Problem:    Cannot read back the prompt of a prompt buffer.
Solution:   Add prompt_getprompt(). (Ben Jackson, closes vim/vim#6851)
https://github.com/vim/vim/commit/077cc7aa0e0c431e97795612374fe17fe7c88803

Updated prompt_getprompt() doc to https://github.com/vim/vim/commit/cb80aa2d53e56d3aba3b3c439fb467f29a750c5e
and removed mention of method syntax usage (not supported by Nvim).

...signs is slow

Problem:    Placing a larger number of signs is slow.
Solution:   Add functions for dealing with a list of signs. (Yegappan
            Lakshmanan, closes #4636)

Problem:    When 'matchpairs' is empty every character beeps. (Marco Hinz)
Solution:   Bail out when no character in 'matchpairs' was found.
            (closes vim/vim#8053)  Add assert_nobeep().
https://github.com/vim/vim/commit/5b8cabfef7c3707f3e53e13844d90e5a217e1e84

Problem:    shiftwidth() does not take 'vartabstop' into account.
Solution:   Use the cursor position or a position explicitly passed.
            Also make >> and << work better with 'vartabstop'. (Christian
            Brabandt)
https://github.com/vim/vim/commit/f951416a8396a54bbbe21de1a8b16716428549f2

Problem:    Cannot parse a date/time string.
Solution:   Add strptime(). (Stephen Wall, closes #)
https://github.com/vim/vim/commit/10455d43fef041309ce0613fa792c635dd71e3a8

N/A patches for version.c:

vim-patch:8.1.2344: Cygwin: warning for using strptime()

Problem:    Cygwin: warning for using strptime().
Solution:   Move defining _XOPEN_SOURCE and __USE_XOPEN to vim.h. (Ken Takata,
            closes vim/vim#5265)  Use 700 for _XOPEN_SOURCE for mkdtemp().
https://github.com/vim/vim/commit/6a228c6463935a73c8f21142cb7368545cfee317

Problem:    It is not easy to change the window layout.
Solution:   Add win_splitmove(). (Andy Massimino, closes vim/vim#4561)
https://github.com/vim/vim/commit/d20dcb3d011da6111153109f6e46fbd5c7fe9fb6