feat: left/right collapsible sidebars

[cguedes]

Closes #68

This PR adds collapsible panels on the left and right.

• The left collapses like VS Code.

  • You click in the "dark sidebar" to toggle what you see (Explorer, References).
  • If you click the active pane collapses (toggle). If collapsed, a click on any sidebar icon will open back the sidebar. The sidebar can also be collapsed by resizing (close and open) the vertical handle.

• The right panel collapses to a widget in the bottom right

  • Closed by clicking a x icon
  • Opens by clicking a ^ chevron icon in the widget
  • Widget has a fixed width

---

[hammer]

Fun! There seems to be some kind of state issue though, as when I first tried this out I collapsed the left sidebar first, then collapsed the right sidebar. When I collapsed the right sidebar, the left sidebar jumped back out at the same time! If I then collapse the left sidebar, they are both collapsed.

I also got it into a state where I can collapse both with one click:

Screen.Recording.2023-06-05.at.9.21.02.PM.mov

[hammer]

@cguedes were the issues I found fixed?

[cguedes]

@hammer not yet. Will work on this now.