'show_line_diagnostics()' and 'show_position_diagnostics()' are
almost identical; they differ only in the fact that the latter also
accepts a column to form a full position, rather than just a line. This
is not enough to justify two separate interfaces for this common
functionality.
Renaming this to simply 'show_diagnostics()' is one step forward, but
that is also not a good name as the '_diagnostics()' suffix is
redundant. However, we cannot name it simply 'show()' since that
function already exists with entirely different semantics.
Instead, combine these two into a single 'open_float()' function that
handles all of the cases of showing diagnostics in a floating window.
Also add a "float" key to 'vim.diagnostic.config()' to provide global
values of configuration options that can be overridden ephemerally.
This makes the float API consistent with the rest of the diagnostic API.
BREAKING CHANGE