Making this opt-out (on by default) was the wrong choice from the
beginning. It is too visually noisy to be enabled by default.

BREAKING CHANGE: Users must opt-in to the diagnostic virtual text
handler by adding

  vim.diagnostic.config({ virtual_text = true })

to their config.

Problem:
Regression from de794f2d2409: `vim.diagnostic.setqflist{open=true}` attempts to
open the location list instead of the diagnostics quickfix list if it didn't
exist before. This is because we are using `qf_id` to decide which to open, but
`qf_id=nil` when there is no existing diagnostics quickfix list with a given
title ("Diagnostics" by default).

Solution:
- Revert to using `loclist` to decide which to open.
- Add tests.

feat(diagnostics)!: sort underline with severity_sort

BREAKING CHANGE: underline will be applied with a higher value than `vim.hl.priorities.diagnostics`

Problem:

Tests have lots of exec_lua calls which input blocks of code
provided as unformatted strings.

Solution:

Teach exec_lua how to handle functions.

This allows the mappings to work with a count and also enables new ]D
and [D mappings to go to the last/first diagnostic in the buffer.