Before the autocommands TextYankPost was not firing with userregs, now
it does. This also fixes the multi-byte encoding of the userreg in the
v:event dict.

fix(filetype): only pass first 100 and last lines to contents check

sufficient for current content checks and avoids performance issues for
buffers with a large number of lines

fixes #19817

docs(lua): update vim.{go,bo,wo} documentation

* document indexing by buffer/window handle
* correct wrapper information (`nvim_buf_{g,s}et_value` now)
* make clear what is considered "invalid key" (consistently)

feat: multibuffer preview support for inccommand

vim-patch:8.2.{partial:0425,0982}: insufficient tests

Problem:    Insufficient testing for reading/writing files.
Solution:   Add more tests. (Yegappan Lakshmanan, closes vim/vim#6257)
            Add "ui_delay" to test_override() and use it for the CTRL-O test.
https://github.com/vim/vim/commit/b340baed9f7fc1c19a0742e2214d54982190c15e

Omit test_override().
Reorder test_writefile.vim to match Vim.

Problem:    Code for modeless selection not sufficiently tested.
Solution:   Add tests.  Move mouse code functionality to a common script file.
            (Yegappan Lakshmanan, closes vim/vim#5821)
https://github.com/vim/vim/commit/515545e11f523d14343b1e588dc0b9bd3d362bc2

Skip termcode tests for now.

vim-patch:8.2.3946,9.0.0227: internal error improvements

Problem:    Cannot read error message when abort() is called.
Solution:   Output a newline before calling abort().
https://github.com/vim/vim/commit/213e70e284b0975dd34525e94e59e26811097c72

Add emsg_not_now() check to make code equivalent.