use boxlayout from lazycore

[jesseduffield]

• PR Description

Lazydocker has historically lagged behind lazygit, and given lazygit has gone through some refactorings that lazydocker has not, it's not always easy to copy functionality from lazygit to lazydocker. I've created the lazycore repo to place shared functionality that can be imported from both projects.

For now it's just going to be an ad-hoc approach but down the line lazycore might become more of a framework than a library and could be used to bootstrap similar projects.

This PR just moves the boxlayout package into lazycore so that lazydocker can allow resizing windows.

• Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go run scripts/cheatsheet/main.go generate)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• Docs (specifically docs/Config.md) have been updated if necessary
• You've read through your own file changes for silly mistakes etc