refactor: preview_auto to use enum and different option

[tessus]

This is a follow-up PR to #1804. It implements an enum and adds a new table [preview] (should be used for max_preview_height and show_preview when the new settings are implemented, although the keys should be renamed then) and the show_preview_auto option has been renamed as mentioned in #991 (comment)

[preview]
strategy = auto (default) | static (max_of_all_cmds)   # was show_preview_auto

However, I am contemplating whether to change the naming:

strategy -> mode
auto -> dynamic

I believe

• mode is used in other settings and thus it might be more consistent.
• dynamic as it determines the height dynamically. auto also fits, so I really have no clue which one is better.

@ellie what do you think?

Checks

• I am happy for maintainers to push small adjustments to this PR, to speed up the review cycle
• I have checked that there are no existing pull requests for the same thing

[ellie]

thank you!

[ellie]

I think mode vs strategy is a bit ambiguous as while we use mode elsewhere, it's not quite the same thing (search mode, filter mode, etc are all for searching history and aren't "smart" decision makers)

Auto seems fine to me

Happy with this as-is

[tessus]

Great. I'll update the docs accordingly later tonight.