Wrap contrib/check-whitespace.jl in a function

[giordano]

This doesn't affect much CI time as the job is vastly dominated by setup rather than performing the actual check, but locally you can enjoy a sensible speedup:

% hyperfine --warmup=1 'julia contrib/check-whitespace.jl' 'julia contrib/check-whitespace-new.jl'
Benchmark 1: julia contrib/check-whitespace.jl
  Time (mean ± σ):      2.143 s ±  0.015 s    [User: 2.880 s, System: 0.056 s]
  Range (min … max):    2.129 s …  2.170 s    10 runs

Benchmark 2: julia contrib/check-whitespace-new.jl
  Time (mean ± σ):      1.080 s ±  0.004 s    [User: 1.830 s, System: 0.050 s]
  Range (min … max):    1.075 s …  1.087 s    10 runs

Summary
  'julia contrib/check-whitespace-new.jl' ran
    1.98 ± 0.02 times faster than 'julia contrib/check-whitespace.jl'

Somewhat ironically, PR better reviewed by ignoring whitespaces.

[giordano]

Note that performing the whitespace check took 4 seconds in this PR, while it took 9 seconds in the parent commit (b8a0a39), confirming the ~2x speedup for running the script.