The overwhelming majority of use cases for `open_float` are to view
diagnostics from the current buffer in a floating window. Thus, most use
cases will just `0` or `nil` as the first argument, which makes the
argument effectively useless and wasteful.
In the cause of optimizing for the primary use case, make the `bufnr`
parameter an optional parameter in the options table. This still allows
using an alternative buffer for those that wish to do so, but makes the
"primary" use case much easier.
The old signature is preserved for backward compatibility, though it can
likely be fully deprecated at some point.
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