RFC: add the ability to disable syntax deprecation warnings

Adds the ability to turn off syntax deprecations in the parser with a command line flag `--no-depwarn` or at runtime:

```julia
julia> a = {1,2,3}
WARNING: deprecated syntax "{a,b, ...}".
Use "Any[a,b, ...]" instead.
julia> a = {1,2,3}

WARNING: deprecated syntax "{a,b, ...}".
Use "Any[a,b, ...]" instead.
3-element Array{Any,1}:
 1
 2
 3

julia> Base.syntax_deprecation_warnings(false)
false

julia> a = {1,2,3}
3-element Array{Any,1}:
 1
 2
 3
```
Can we add a comment that this mirrors the `jl_compileropts_t` struct from julia.h