Support const syntax for mutable structs in lazy field

[Tortar]

It doesn't seem to work currently:

julia> @lazy mutable struct A
           const x::Int
           @lazy const y::Int
ERROR: ParseError:
# Error @ REPL[12]:3:11
    const x::Int
    @lazy const y::Int
#         └──────────┘ ── expected assignment after `const`
Stacktrace:
 [1] top-level scope
   @ none:1

actually given this is a parse error, it is probably necessary to go instead with

julia> @lazy mutable struct A
           const x::Int
           const @lazy y::Int
       end
ERROR: LoadError: expected a @lazy field inside the struct
Stacktrace:
 [1] error(s::String)
   @ Base ./error.jl:35
 [2] lazy_struct(expr::Expr, mod::Module)
   @ LazilyInitializedFields ~/.julia/packages/LazilyInitializedFields/EAhdM/src/LazilyInitializedFields.jl:307
 [3] var"@lazy"(__source__::LineNumberNode, __module__::Module, expr::Any)
   @ LazilyInitializedFields ~/.julia/packages/LazilyInitializedFields/EAhdM/src/LazilyInitializedFields.jl:264
in expression starting at REPL[13]:1

This is useful when you don't really want to initialize always that field in all cases, but only for some instances, but if it is initiliazed to an actual value you want that it can't be changed (as in my case)