Detect splatting kwarg prep

[timholy]

We were failing to detect kw functions that accepted vararg positional args.

[codecov]

Codecov Report

Merging #403 into master will decrease coverage by 0.00%.
The diff coverage is 88.00%.

@@            Coverage Diff             @@
##           master     #403      +/-   ##
==========================================
- Coverage   87.72%   87.72%   -0.01%     
==========================================
  Files          11       11              
  Lines        2013     2037      +24     
==========================================
+ Hits         1766     1787      +21     
- Misses        247      250       +3     

Impacted Files	Coverage Δ
src/utils.jl	84.93% <84.61%> (-0.02%)	⬇️
src/commands.jl	92.98% <91.66%> (-0.11%)	⬇️

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[timholy]

Any objections?

[KristofferC]

Didn't look too carefully but the test shows it works so LGTM. It is a bit unfortunate that we need to have so much complicated code to match but it is what it is. Perhaps in the future we could think about if the compiler could emit som helpful markers in the lowered code or something to help with stuff like this.

[timholy]

I think the ultimate answer is JuliaLang/julia#31162, but perhaps in its absence we could come up with something else?