Triple-quoted var does not work

Using the `var"..."` syntax to construct a variable fails when using triple quotes, i.e. `var"""..."""`. This is a regression compared to the previous parser so I believe this is a parsing problem that belongs here, but let me know if I should open this issue on the main Julia repo instead.

In julia v1.9 and before:

```julia
julia> var"""x""" = 12
12

julia> x
12
```

whereas in julia v1.10 and later:

```julia
julia> var"""x""" = 12
ERROR: LoadError: UndefVarError: `@var_str` not defined
in expression starting at REPL[1]:1
```