Fix floating-point arithmetic issues when props are decimals.

[jcfrancisco]

Consider the following case:

      const value = 5.3;
      const props = {
        marks: {},
        step: 0.05,
        min: 0,
        max: 5.3
      };

I would expect the output of getClosestPoint to be 5.3. However, it returns 5.25.

This is because of floating point arithmetic issues with the function.

I've added a test case and taken a stab at figuring it out, making sure I continue to respect the Math.floor logic on maxSteps.

[jcfrancisco]

This function is untouched, moved it up because I'm using it elsewhere.

[jcfrancisco]

I can back out this refactor if it's unwanted

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.83%. Comparing base (5ba4181) to head (e4768ef).
⚠️ Report is 219 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #619      +/-   ##
==========================================
+ Coverage   95.40%   95.83%   +0.43%     
==========================================
  Files           2        2              
  Lines          87       96       +9     
  Branches       29       30       +1     
==========================================
+ Hits           83       92       +9     
  Misses          4        4              

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