Print var"string" as raw string (#57887)
Fixes #57076. As stated in a TODO comment, `show_sym` prints `var""`
symbols as ordinary strings (where [many
things](https://github.com/JuliaLang/julia/blob/master/base/strings/io.jl#L485-L491)
are escaped), but it should be a raw string (where only quotes and some
backslashes are escaped). This fails the eval-parse-repr printing test.
On master:
```
julia> a = :(var"\t,\$,\",\\,\u03c0," = 1)
:(var"\\t,\\\$,\",\\\\,\\u03c0," = 1)
julia> a = eval(Meta.parse(repr(a)))
:(var"\\\\t,\\\\\\\$,\",\\\\\\\\,\\\\u03c0," = 1)
julia> a = eval(Meta.parse(repr(a)))
:(var"\\\\\\\\t,\\\\\\\\\\\\\\\$,\",\\\\\\\\\\\\\\\\,\\\\\\\\u03c0," = 1)
```
This PR:
```
julia> a = :(var"\t,\$,\",\\,\u03c0," = 1)
:(var"\t,\$,\",\\,\u03c0," = 1)
julia> a = eval(Meta.parse(repr(a)))
:(var"\t,\$,\",\\,\u03c0," = 1)
```
---------
Co-authored-by: Keno Fischer <keno@alumni.harvard.edu>
