support for const fields of Julia 1.8?

[mind6]

Currently if using 1.8RC3, the following gives a parse error:

mutable struct S
    const x::Int
    y::Int
end

But it's allowed in 1.8 as one of the new language features: https://github.com/JuliaLang/julia/blob/v1.8.0-beta1/NEWS.md

[pfitzseb]

This is fixed on master via julia-vscode/CSTParser.jl#337 and #336