[Resize content][2/n] Implement ResizableSplitView

[taneliang]

Stack PR by STACK ATTACK:

• [Resize content][1/n] Break horizontal scrolling area into final-ish hierarchy #110 [Resize content][1/n] Break horizontal scrolling area into final-ish hierarchy
• [Resize content][2/n] Implement ResizableSplitView #107 [Resize content][2/n] Implement ResizableSplitView
• [Resize content][3/n] Sync horizontal pan and zoom states #108 [Resize content][3/n] Sync horizontal pan and zoom states
• Implement User Timing marks view #109 Implement User Timing marks view
• Highlight React events with the same wakeable ID #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 :)

[kartikcho]

Is it really necessary to introduce a dependency for a simple helper function? I think we can just write a function to accommodate for it here.

[taneliang]

We can definitely declare it ourselves, but since this nullthrows package is used in the React monorepo as well I figured it's okay to just add it as a dependency.

[kartikcho]

Yeah, that makes sense.

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[taneliang]

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