Allow while-block in at-animate macro

[giordano]

The @animate example in documentation just works for me on this branch with

anim = @animate while i ≤ n
    circleplot(x, y, i)
    i += 1
end

For reference: https://discourse.julialang.org/t/animate-with-while-loop-in-julia/61069

[codecov]

Codecov Report

Merging #3497 (7c1c572) into master (d544647) will decrease coverage by 0.01%.
The diff coverage is 50.00%.

❗ Current head 7c1c572 differs from pull request most recent head 788e28d. Consider uploading reports for the commit 788e28d to get more accurate results

@@            Coverage Diff             @@
##           master    #3497      +/-   ##
==========================================
- Coverage   64.13%   64.11%   -0.02%     
==========================================
  Files          27       27              
  Lines        6610     6610              
==========================================
- Hits         4239     4238       -1     
- Misses       2371     2372       +1     

Impacted Files	Coverage Δ
src/animation.jl	38.20% <50.00%> (ø)
src/types.jl	47.82% <0.00%> (-4.35%)	⬇️

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[daschw]

Nice, thanks!