Problem:
nvim_buf_del_extmark error if buffer is destroyed before timer stops

Solution:
check nvim_buf_is_valid.

Problem: Currently vim.hl.range only allows one timed highlight.
Creating another one, removes the old one.

Solution: vim.hl.range now returns a timer and a function. The timer
keeps track of how much time is left in the highlight and the function
allows you to clear it, letting the user decide what to do with old
highlights.

Problem: yi' don't highlight last character since
https://github.com/neovim/neovim/commit/8ce504820af04194a41acbe1f4c61cf12bd5feb5.

Solution: Always use `opts.inclusive=true`, since calculation of `"]`
(`b_op_end`) have taken `inclusive` into account.

Problem:
`vim.hl.on_yank()` has a "timeout" behavior but this is not available for
`vim.hl.range()`.

Solution:
Add `timeout` arg to `vim.hl.range()`.

Problem:

`vim.validate()` takes two forms when it only needs one.

Solution:

- Teach the fast form all the features of the spec form.
- Deprecate the spec form.
- General optimizations for both forms.
- Add a `message` argument which can be used alongside or in place
  of the `optional` argument.

Problem:
- `vim.highlight` module does not follow `:help dev-name-common`, which
  documents the name for "highlight" as "hl".
- Shorter names are usually preferred.

Solution:
Rename `vim.highlight` to `vim.hl`.

This is not a breaking change until 2.0 (or maybe never).