Add support of `ignore` tag for `json.marshal`

[VladPavliuk]

Propose an ability to specify optional ignore tag for struct fields to exclude them from json.marshal json object output.

Example:

My_Struct :: struct {
    a: string `json:"-"`,
}

my_struct := My_Struct{
    a = "test",
}

marshaled_data, _ := json.marshal(my_struct)

will produce:

{}

[laytan]

IMO it would be better to do json:"-" to match cbor:"-" and fmt:"-" which do the same thing in their respective packages.

[gingerBill]

Please use - not _.

[VladPavliuk]

@laytan @gingerBill thanks for pointing that out! I didn't know about that already existing ignore functionality in cbor and fmt. The changes where pushed to the branch.