Provide a 'format' option for virtual text and floating window previews
that allows the displayed text of a diagnostic to be customized.

This function isn't compatible with including diagnostic sources when
"source" is "if_many" since it only has access to diagnostics for a
single line. Rather than having an inconsistent or incomplete interface,
make this function private. It is still exported as part of the module
for backward compatibility with vim.lsp.diagnostics, but it can
eventually be made into a local function.

Add an option to virtual text display and floating window previews to
include diagnostic source in the diagnostic message.

Some parts of LSP need to use cached diagnostics as sent from the LSP
server unmodified. Rather than fixing invalid line numbers when
diagnostics are first set, fix them when they are displayed to the user
(e.g. in show() or one of the get_next/get_prev family of functions).

* 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 vim.diagnostic.config() is called, the decorations for diagnostics
are re-displayed to use the new configuration. This should only be done
for loaded buffers.

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.