Problem:
Diagnostic positions are not being updated after text changes, which
means `vim.diagnostic.open_float` and `vim.diagnostic.jump` will work
with outdated positions when text is changed until diagnostics are
updated again (if ever).
Solution:
Create extmarks in `vim.diagnostic.set` and use their positions for
`vim.diagnostic.open_float` and `next_diagnostic` (used by
`vim.diagnostic.jump`, `vim.diagnostic.get_next` and
`vim.diagnostic.get_prev`).
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