feat(handlebars plugin): adding handlebars helpers for common math/formatting operations

[rusackas]

SUMMARY

Adds the just-handlebars-helpers module to further expand the capabilities of the Handlebars viz plugin, allowing you to do common math operations and such in your templating

Hat tip to @villebro who did the real legwork on this long ago... I'm just making sure it gets into the codebase for all to enjoy :)

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

N/A - but the world is your oyster :)

TESTING INSTRUCTIONS

Just give it a whirl! On the ephemeral environment of this PR, or by building something fun on your own.

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #20648 (161c6ce) into master (e331640) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master   #20648      +/-   ##
==========================================
- Coverage   66.78%   66.78%   -0.01%     
==========================================
  Files        1754     1754              
  Lines       65857    65858       +1     
  Branches     7030     7030              
==========================================
  Hits        43982    43982              
- Misses      20086    20087       +1     
  Partials     1789     1789              

Flag	Coverage Δ
hive	53.88% <ø> (ø)
javascript	51.93% <0.00%> (-0.01%)	⬇️
mysql	82.36% <ø> (ø)
postgres	82.43% <ø> (ø)
presto	53.74% <ø> (ø)
python	82.87% <ø> (ø)
sqlite	82.22% <ø> (ø)
unit	50.79% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...ars/src/components/Handlebars/HandlebarsViewer.tsx	0.00% <0.00%> (ø)

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[ShadowA7X]

I'd like to highlight the addition of the just-handlebars-helpers module in this PR. This module, found here: https://github.com/leapfrogtechnology/just-handlebars-helpers, provides a comprehensive set of helpers which enhances the capabilities of the Handlebars plugin in Superset. Its integration will significantly extend the templating functionalities available in Superset's Handlebars plugin.