Compute the diagnostics per line when `show` is called, allowing for
O(1) access for the diagnostics to display when the cursor line or the
list of diagnostics haven't changed.

Problem:
The current implementation uses a global augroup for virtual lines in
diagnostics, which can lead to conflicts and unintended behavior when
multiple namespaces/buffers are involved.

Solution:
Refactor the code to use a namespace-specific augroup for virtual lines.
This ensures that each namespace has its own augroup.
Scope the clear commands to only the relevant buffer.

Problem: Right aligned virtual text can cover up buffer text if virtual
text is too long

Solution: An additional option for `virt_text_pos` called
`eol_right_align` has been added to truncate virtual text if it would
have otherwise covered up buffer text. This ensures the virtual text
extends no further left than EOL.

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.