Later Vim patches changed to return 0 for empty string and null string.
Also update setcellwidth() docs to match latest Vim.

vim-patch:8.2.1536: cannot get the class of a character; emoji widths are wrong

Problem:    Cannot get the class of a character; emoji widths are wrong in
            some environments.
Solution:   Add charclass(). Update some emoji widths.  Add script to check
            emoji widths.
https://github.com/vim/vim/commit/4e4473c927167fd24e5c8df90e0e8035080cf2da

Use latest charclass() docs from Vim.
Rewrite DoIt() in emoji_list.vim in Lua.
Omit emoji table updates:
- emoji_width update looks wrong as these added ranges are only double-width when followed by 0xFE0F.
- Other updates are too old.

vim-patch:8.2.1540: the user cannot try out emoji character widths

Problem:    The user cannot try out emoji character widths.
Solution:   Move the emoji script to the runtime/tools directory.
https://github.com/vim/vim/commit/98945560c1ae6e2ddee820a7de718a36e3f4b6e5

Problem:    various code is not properly tested.
Solution:   Add more tests. (Yegappan Lakshmanan, closes vim/vim#5871)
https://github.com/vim/vim/commit/cde0ff39da2459b16007fef701ebaa449fb6fe9d

Cherry-pick test_clientserver.vim change from patch 8.1.1826.
Cherry-pick a comment from patch 8.2.0301.
Omit test_viminfo.vim.

Problem:    Not enough testing for movement commands.
Solution:   Add more tests. (Yegappan Lakshmanan, closes vim/vim#6313)
https://github.com/vim/vim/commit/bdd2c290d3cda69e0046c42f0c651f60bc510a16

Cherry-pick test_functions.vim changes from patch 8.2.0183.
Cherry-pick Test_normal18_z_fold() change from patch 8.2.0540.

Problem:    Ex_getln.c code not covered by tests.
Solution:   Add a few more tests. (Yegappan Lakshmanan, closes vim/vim#5702)
https://github.com/vim/vim/commit/578fe947e3ad0cc7313c798cf76cc43dbf9b4ea6

Cherry-pick Test_Ex_global() from patch 8.2.0293.
Test_rightleftcmd() fails if incsearch is enabled, so disable it.

Problem:    input() does not handle composing characters properly.
Solution:   Use mb_cptr2char_adv() instead of mb_ptr2char_adv().
            (closes vim/vim#10527)
https://github.com/vim/vim/commit/e3a529bc877909a9eccf792461050b4f6737ed33

Cherry-pick all of Test_input_func() from patch 8.2.0316.

Problem:    K_SPECIAL may be escaped twice.
Solution:   Avoid double escaping. (closes vim/vim#10340)
https://github.com/vim/vim/commit/db08887f24d20be11d184ce321bc0890613e42bd

Problem:    delete() with "rf" argument does not report a failure.
Solution:   Return -1 if the directory could not be removed. (closes vim/vim#10078)
https://github.com/vim/vim/commit/478700336d1c72e133b8ff6841e968c1bb1658ed

Problem:    Delete() can not handle a file name that looks like a pattern.
Solution:   Use readdir() instead of appending "/*" and expanding wildcards.
            (Ken Takata, closes vim/vim#4424, closes vim/vim#696)
https://github.com/vim/vim/commit/701ff0a3e53d253d7300c385e582659bbff7860d

Cherry-pick a change to Test_delete_rf() from patch 8.1.1921.

Co-authored-by: zeertzjq <zeertzjq@outlook.com>

Problem:    getmousepos() does not compute the column below the last line.
Solution:   Also compute the column when the mouse is below the last line.
            (Sean Dewar, closes vim/vim#9946)
https://github.com/vim/vim/commit/10792feebd237aee89270669e509e85cafdfac60

test_setmouse is N/A.