The prefix option of the diagnostic virtual text can be a function,
but previously it was only a function of diagnostic.

This function should also have additional parameters index and total,
more consistently and similarily as in the prefix function for
`vim.diagnostic.open_float()`.

These additional parameters will be useful when there are too many
number of diagnostics in a single line.

Allow users to filter diagnostics by specifying severities

- vim.diagnostic.config() now accepts a function for the virtual_text.prefix
  option, which allows for rendering e.g., diagnostic severities differently.

There is already a call to clear() in before_each(), so after_each() isn't necessary.

The LSP spec supports two tags that can be added to diagnostics:
unnecessary and deprecated. Extend vim.diagnostic to be able to handle
these.

The existing groups, Error, Hint, Info, Warn cover many use cases, but
neglect the occasion where a diagnostic message should communicate a
non-informative (not a Hint or Info) event. DiagnosticOk covers this
with a generic green colorscheme.

The BufWipeout autocmd is not 100% reliable and may leave stale entries
in the cache. This is sort of a hack/workaround to ensure
`vim.diagnostic.reset` calls don't fail if there are stale cache entries
but instead clears them

Fixes errors like

    Error executing vim.schedule lua callback: /usr/share/nvim/runtime/lua/vim/diagnostic.lua:1458: Invalid buffer id: 22
    stack traceback:
            [C]: in function 'nvim_exec_autocmds'
            /usr/share/nvim/runtime/lua/vim/diagnostic.lua:1458: in function 'reset'

This introduces a `suffix` option to the `virt_text` config in
`vim.diagnostic.config()`. The suffix can either be a string which is appended
to the diagnostic message or a function returning such. The function receives a
`diagnostic` argument, which is the diagnostic table of the last diagnostic (the
one whose message is rendered as virt text).