[Resize content][2/n] Implement ResizableSplitView

[taneliang]

Stack PR by STACK ATTACK:

• 110 [Resize content][1/n] Break horizontal scrolling area into final-ish hierarchy

• #107 [Resize content][2/n] Implement ResizableSplitView

• 108 [Resize content][3/n] Sync horizontal pan and zoom states

• 109 Implement User Timing marks view

• 111 Highlight React events with the same wakeable ID

Summary

Implements ResizableSplitView and uses it to allow resizing of the React measures view.

This PR also fixes the last PR's known issue where the flamechart does not scroll vertically.

Resolves #19, resolves #70.

Test Plan

• yarn lint
• yarn flow: no errors in changed code
• yarn test: no added tests, but everything still passes
• yarn start: manual testing:
  • Hover over resize bar to observe color change
  • Drag resize bar to resize
  • Resize bar position is clamped and cannot go offscreen or expand beyond the size of the lanes area
  • Content is independently vertically scrollable
  • Resize bar remains visible on page resize

There are still 3 independent horizontal pan/zoom areas, a known issue carried over from the last PR in this stack. This will be fixed in the next PR (#108).

[taneliang]

I'll decrease the bar's height tomorrow. @kartikcho I'd appreciate it if you could approve this PR first so that I can land everything tomorrow morning :)

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[taneliang]

Decreased bar height by 1px (it's now 5px tall) and rebased on master