Use nvim_exec_autocmds to issue the DiagnosticChanged autocommand,
rather than nvim_buf_call, which has some side effects when drawing
statuslines.

Closes https://github.com/neovim/neovim/issues/13647

This allows customizing the priority of the highlights.

* Add default priority of 50
* Use priority of 200 for highlight on yank
* use priority of 40 for highlight references (LSP)

Function arguments that expect a list should explicitly use tbl_islist
rather than just checking for a table. This helps catch some simple
errors where a single table item is passed as an argument, which passes
validation (since it's a table), but causes other errors later on.

The overwhelming majority of use cases for `open_float` are to view
diagnostics from the current buffer in a floating window. Thus, most use
cases will just `0` or `nil` as the first argument, which makes the
argument effectively useless and wasteful.

In the cause of optimizing for the primary use case, make the `bufnr`
parameter an optional parameter in the options table. This still allows
using an alternative buffer for those that wish to do so, but makes the
"primary" use case much easier.

The old signature is preserved for backward compatibility, though it can
likely be fully deprecated at some point.

Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Co-authored-by: Gregory Anders <greg@gpanders.com>

Closes #16453

Co-authored-by: Cédric Barreteau <cbarrete@users.noreply.github.com>