Fix printing of Vararg on Julia 1.7

[jakobnissen]

Recent versions of Julia has moved Vararg from being a Type to being its own thing. This means it will error if T is a vararg and you try to do isa to <: operations with it. This caused an error, which is fixed here by explicitly testing if T is a Vararg.

Fixes #261

Note that THIS IS NOT TESTED! I couldn't figure out how to write a test for this.

[Roger-luo]

could you just copy-paste the example from that issue in the test? that should be sufficient as a test

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
src/ast/printing.jl	84.69% <80.00%> (+84.69%)	:arrow_up:

... and 22 files with indirect coverage changes

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