refactor: rename fn.bind to fn.bind_ to strongly indicate it should not be used externally

[misteroneill]

Description

The utils/fn.bind function will be exposed in Video.js 8 because fn is exposed as videojs.fn. We don't want people using it, but it cannot be removed without breaking the Video.js event system.

This PR adds a _ at the end of its name to indicate that it is private.

No external updates should be needed as it was not directly exposed. The previously-supported videojs.bind has been deprecated.

Specific Changes proposed

• Rename utils/fn.bind to utils/fn.bind_
• Update usages

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Change has been verified in an actual browser (Chrome, Firefox, IE)
  • Unit Tests updated or fixed
  • Docs/guides updated
  • Example created (starter template on JSBin)
• Reviewed by Two Core Contributors

[gkatsev]

Rename to Fn.bind_but_dont_use_or_you_will_be_fired

[codecov]

Codecov Report

Merging #7940 (bc91650) into next (b5fefde) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             next    #7940      +/-   ##
==========================================
+ Coverage   81.96%   81.97%   +0.01%     
==========================================
  Files         111      111              
  Lines        7356     7356              
  Branches     1776     1776              
==========================================
+ Hits         6029     6030       +1     
+ Misses       1327     1326       -1     

Impacted Files	Coverage Δ
src/js/component.js	94.11% <100.00%> (ø)
...control-bar/progress-control/mouse-time-display.js	63.63% <100.00%> (ø)
.../control-bar/progress-control/play-progress-bar.js	88.23% <100.00%> (ø)
...s/control-bar/progress-control/progress-control.js	26.66% <100.00%> (ø)
src/js/control-bar/progress-control/seek-bar.js	55.19% <100.00%> (ø)
...rc/js/control-bar/progress-control/time-tooltip.js	84.61% <100.00%> (+2.56%)	⬆️
...rol-bar/text-track-controls/descriptions-button.js	100.00% <100.00%> (ø)
src/js/control-bar/track-button.js	94.11% <100.00%> (ø)
...l-bar/volume-control/mouse-volume-level-display.js	53.84% <100.00%> (ø)
...rc/js/control-bar/volume-control/volume-control.js	48.78% <100.00%> (ø)
... and 9 more

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