Proposal/Request: Expose `False()` and `True()` constructors

**Is your feature request related to a problem? Please describe.**
While using schema inference is nice, sometimes I need to define a jsonschema "manually" in Go code. In those cases, it's common to need to define certain fields
of the schema as either `true` or `false`.

With the current interface, this looks like this:
```
schema := &jsonschema.Schema{
Type: "object",
Properties: map[string]*jsonschema.Schema{
"field": {
Type: "string",
},
},
// We want AdditionalProperties: false
AdditionalProperties: &jsonschema.Schema{Not: &jsonschema.Schema{}},
},
```

Not only is that hard to read, but it's not obvious to most users that `&jsonschema.Schema{Not: &jsonschema.Schema{}}` is equivalent to `false` in the spec.

**Describe the solution you'd like**
I think it'd be great to have two publicly available constructors:
```
func True() *Schema {
return &Schema{}
}

func False() *Schema {
return &Schema{Not: &Schema{}}
}
```

That would make it possible to write:
```
schema := &jsonschema.Schema{
Type: "object",
Properties: map[string]*jsonschema.Schema{
"field": {
Type: "string",
},
},
AdditionalProperties: jsonschema.False(),
},
```

**Describe alternatives you've considered**
For now I've implemented `True()` and `False()` in my own code, but I'm hoping
these can be part of `jsonschema`.

**Additional context**
I'm happy to put together the PR and send it for review if this proposal is approved.