Problem:    Incorrect error messages for functions that now take a Blob
            argument.
Solution:   Adjust the error messages. (Dominique Pelle, closes vim/vim#3846)
https://github.com/vim/vim/commit/0d17f0d1c09fa6db306336695ba646c21ea24909

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.

An empty table was previously always treated as a list, which means that
while merging tables, whenever an empty table was encountered it would
  always truncate any table on the left.

`vim.tbl_deep_extend("force", { b = { a = 1 } }, { b = {} })`

Before: `{ b = {} }`
After: `{ b = { a = 1 } }`

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>

This closes #14677, but I also am a little unsure if there are times
where this may not be correct. However, this just changes the behavior
that even if `was_set` was false, we still get for
`nvim_win_get_option`.