Closes https://github.com/blopker/codebook/issues/79

Recently, the setting `diagnostics_max_severity` was changed from `null`
to `warning`in this PR: https://github.com/zed-industries/zed/pull/30316
This change has caused the various spell checking extensions to not work
as expected by default, most of which use the `hint` diagnostic. This
goes against user expectations when installing one of these extensions.

Without `hint` as the default, extension authors will either need to
change the diagnostic levels, or instruct users to add
`diagnostics_max_severity` to their settings as an additional step,
neither of which is a great user experience.

This PR sets the default `hint`, which is closer to the original
behavior before the aforementioned PR.

Release Notes:

- Changed `diagnostics_max_severity` to `hint` instead of `warning` by
default

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Co-authored-by: Kirill Bulatov <kirill@zed.dev>