The test added in 274a3504a790a799b28ee89c75e29fb4dbdff41f
does not fail if the code changes are reverted.

Resolve an issue with deferred clearing of highlight failing if the
buffer is deleted before the timeout by checking whether the
buffer is valid first.

Analogous to nodejs's `on('data', …)` interface, here on_key is the "add
listener" interface.

ref 3ccdbc570d85 #12536

BREAKING_CHANGE: vim.register_keystroke_callback() is now an error.

Options formatted as a list of comma-separated key-value pairs may have
values that contain leading and trailing whitespace characters. For
example, the `listchars` option has a default value of
`"tab:> ,trail:-,nbsp:+"`. When converting this value to a lua table,
leading and trailing whitespace should not be trimmed.

Co-authored-by: Robert Hrusecky <robert.hrusecky@utexas.edu>

An assortment of various vim.opt fixups.

Extmarks: manually zero out `curbuf->deleted_bytes2` on substitute and join