Add support for controlling text alignment in DataFrame and Tabulator

[philippjfr]

As the title says, this PR provides control over the text alignment of columns of the DataFrame and Tabulator widgets.

[codecov]

Codecov Report

Merging #2331 (651edc3) into master (a6dbf03) will decrease coverage by 0.02%.
The diff coverage is 55.55%.

@@            Coverage Diff             @@
##           master    #2331      +/-   ##
==========================================
- Coverage   84.10%   84.08%   -0.03%     
==========================================
  Files         181      181              
  Lines       22064    22073       +9     
==========================================
+ Hits        18557    18560       +3     
- Misses       3507     3513       +6     

Impacted Files	Coverage Δ
panel/widgets/tables.py	83.67% <55.55%> (-0.41%)	⬇️
panel/io/reload.py	64.36% <0.00%> (-2.30%)	⬇️

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