Introduce `stable_size_docks` setting

[hron]

Currently, Zed maintains the size for every panel separately. This is probably what most of the users want, but it can be really annoying when the layout changes unexpectedly. The stable-size-docks feature allows users to define a fixed size for each dock, ensuring a consistent layout when switching panels.

https://github.com/user-attachments/assets/ef527082-8234-4ed5-9f8e-0f4ae641b295

Release Notes:

• Added stable_size_docks configuration, which allows to keep the same size for all panels in specific dock

[niekdomi]

Not sure, but I think this is similar to this PR: https://github.com/zed-industries/zed/pull/30551

nevermind, already got mentioned

[mikayla-maki]

I think I'm going to close this one in favor of https://github.com/zed-industries/zed/pull/30551, as some panels (e.g. the Agent panel) want very different default sizes than others (e.g. the collab panel)

[mikayla-maki]

Thank you for the contribution though!

[hron]

@mikayla-maki the solution you chose works well for me as well, so I'm happy the issue is resolved even if my PR wasn't the best fit :)