Add closest bound to onBeforeChange event

[lutskanu]

Add closest bound identifier to onBeforeChange event (0 for left and 1 for right).

[codecov-io]

Codecov Report

Merging #498 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #498   +/-   ##
=======================================
  Coverage   82.86%   82.86%           
=======================================
  Files          10       10           
  Lines         461      461           
  Branches      118      118           
=======================================
  Hits          382      382           
  Misses         79       79

Impacted Files	Coverage Δ
src/Range.jsx	60.13% <ø> (ø)	:arrow_up:
src/Slider.jsx	100% <ø> (ø)	:arrow_up:

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[yoyo837]

Rebase please.