Implement reveal.js based SlideTemplate

[philippjfr]

Working on a talk and decided that I no longer wanted to use RISE, so I decided to implement a slides template for Panel instead.

[codecov]

Codecov Report

Merging #4798 (50acf8d) into main (0736250) will increase coverage by 12.53%.
The diff coverage is 97.56%.

@@             Coverage Diff             @@
##             main    #4798       +/-   ##
===========================================
+ Coverage   70.67%   83.21%   +12.53%     
===========================================
  Files         268      269        +1     
  Lines       37943    37972       +29     
===========================================
+ Hits        26818    31600     +4782     
+ Misses      11125     6372     -4753     

Flag	Coverage Δ
ui-tests	39.72% <78.04%> (?)
unitexamples-tests	73.35% <97.56%> (+2.67%)	⬆️

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Impacted Files	Coverage Δ
panel/template/golden/__init__.py	86.36% <83.33%> (+19.69%)	⬆️
panel/template/__init__.py	100.00% <100.00%> (ø)
panel/template/base.py	78.52% <100.00%> (+0.13%)	⬆️
panel/template/bootstrap/__init__.py	100.00% <100.00%> (ø)
panel/template/fast/base.py	95.00% <100.00%> (ø)
panel/template/material/__init__.py	100.00% <100.00%> (ø)
panel/template/slides/__init__.py	100.00% <100.00%> (ø)
panel/template/vanilla/__init__.py	100.00% <100.00%> (+8.33%)	⬆️
panel/viewable.py	66.16% <100.00%> (+2.34%)	⬆️

... and 67 files with indirect coverage changes

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[philippjfr]

Will merge and add docs later. Want to get a last RC out.