Support "write" access mode

[mingyc]

Motivation and details are in #175

• At least two implementers are interested (and none opposed):
  • This is a Chromium proposal
  • …
• Tests are written and can be reviewed and commented upon at:
  • …
• Implementation bugs are filed:
  • Chromium: https://chromestatus.com/feature/5324970347200512
  • Gecko: …
  • WebKit: …
• MDN issue is filed: Add 'write' value to FileSystemHandle permission descriptor.mode mdn/content#40535
• The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

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