Support write-only handles

[foolip]

The inline issue in https://wicg.github.io/native-file-system/#api-filesystemhandle-querypermission states:

the currently described API here assumes a model where it is not possible to have a write-only handle. I.e. it is not possible to have or request write access without also having read access. There definitely are use cases for write-only handles (i.e. directory downloads), so we might have to reconsider this.

#29 is about one use case for this, but I couldn't find an issue that would cover write-only directory downloads, so here it is.

[jromy5]

The changing of the API file code

[mingyc]

Started a proposal to add write mode: #463