allow `Meta.parse` to take optional `filename` argument (#50224)
Code analysis tools such as JET.jl use `Meta.parse` to get a Julia code representation from
textual data, or to display diagnostic messages when the input data is invalid. In such
scenario it would be beneficial if these diagnostic messages could refer to the original
file name of the input data. This commit allows `Meta.parse` to accept the optional
`filename::AbstractString` argument for this purpose. If an error occurs, the `filename` can
be included in the diagnostic information. The default value is set to `"none"`, thus
preserving existing behavior.
