feat: make side panels resizable

[Kiruel]

Adding resize windows for widgetbook shell.

List of issues which are fixed by the PR

fix #621

Screenshots

If applicable, add screenshots to help explain the changes.

Checklist

• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making].
• All existing and new tests are passing.

If you need help, consider asking for advice on Discord.

[docs-page]

To view this pull requests documentation preview, visit the following URL:

docs.widgetbook.io/~738

Documentation is deployed and generated using docs.page.

[YoussefRaafatNasry]

Great work contributing to fix this! 🎉

Although this PR uses an external package, which is something we really not in fan of lately after removing most of our dependencies (bloc, provider, freezed, etc.). But I can see that the resizable_widget package:

1. is somehow stable and doesn't get a lot of updates.
2. is not commonly used by mobile app developers (the big percent of Flutter users), so it won't result in version conflicts.

[YoussefRaafatNasry]

@Kiruel I modified this code a bit as follows:

1. Sort children to be the last parameter for more readability.
2. Remove Material widget; You probably ran into this issue because you've used the hosted version of widgetbook_core and not the local one. We use melos to manage dependencies, so consider using melos bs instead of flutter pub get in your future PRs.