* feat(diagnostic): add vim.diagnostic.match()
Provide vim.diagnostic.match() to generate a diagnostic from a string and
a Lua pattern.
* feat(diagnostic): add tolist() and fromlist()
When severity_sort is true, higher severities should be displayed before
lower severities (e.g. ERROR is displayed over WARN).
Also improved the test case for this.
This generalizes diagnostic handling outside of just the scope of LSP.
LSP clients are now a specific case of a diagnostic producer, but the
diagnostic subsystem is decoupled from the LSP subsystem (or will be,
eventually).
More discussion at [1].
[1]: https://github.com/neovim/neovim/pull/15585
