Add format check

[t-bltg]

Add a formatting check action.

Rationale:

• avoid polluting git history with format PRs, developers should be advised to run using JuliaFormatter; format(["src", "test"]) before the PR is merged.
• keep the format PR action in case some unformatted changes slip through, only run it less often (monthly instead of weekly).

[codecov]

Codecov Report

Merging #4053 (61c05e1) into master (f333cb2) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #4053      +/-   ##
==========================================
- Coverage   62.13%   62.11%   -0.02%     
==========================================
  Files          29       29              
  Lines        7286     7286              
==========================================
- Hits         4527     4526       -1     
- Misses       2759     2760       +1     

Impacted Files	Coverage Δ
src/backends/unicodeplots.jl	51.06% <0.00%> (ø)
src/types.jl	47.82% <0.00%> (-4.35%)	⬇️
src/backends/gr.jl	88.27% <0.00%> (-0.09%)	⬇️
src/recipes.jl	58.60% <0.00%> (+0.11%)	⬆️

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