[6.x] Resizable sidebar

[jaygeorge]

What this PR Does

This is part of the new forms experience, which has 2 resizable sidebars.
The "left" sidebar could be used anywhere in the CP at the moment, so I'm making this PR.

This PR:

• Renames nav-main to cp-sidebar-start. Since we'll have "left" and "right" sidebars, this makes more sense. I could have called it cp-sidebar-left, but I wanted to think ahead and use logical naming conventions for RTL support. FYI the second bar will be called cp-sidebar-end.
• Makes the sidebar resizable in Vue, with a sensible min/max width
• You can always double-click the resize bar to reset its size

How to Reproduce

1. On any page hover over the line that divides the sidebar and the main content
2. Drag around to observe the min/max values in place
3. Double click to reset the navbar size

[duncanmcclean]

Replaced by #14378