Add the FileSystemFileHandle.move method

[valadaptive]

An earlier version of this was proposed in #10, but stalled out due to complications around moving directories.

When looking into it, I was therefore surprised to find that this method (at least, a subset of it) has shipped in browsers since late 2021!

Apparently, Firefox and WebKit both shipped (early versions of) this method without a flag, and have kept them ever since. Chromium added these methods to the public IDL of FileSystemFileHandle in a commit that claimed only to remove them from FileSystemHandle. (A slightly-later update changed the single-argument version from rename to move.)

I've taken #10 and adjusted it to match what Chrome has actually been shipping, in the hope that it can be standardized and documented. In particular, directory entries can no longer be moved, and the move method has been moved from FileSystemHandle to FileSystemFileHandle.

Aside from just moving things around in the spec, there are a couple other changes:

• I've reworded the non-normative sections slightly, since Bikeshed now warns if you use the word "may" in a non-normative section.

• I've removed a couple steps in the algorithm that only exist to handle edge cases around moving directories. These steps were ensuring a directory could not be moved within itself, and ensuring the root of a bucket file system could not be moved.

The rewording is in its own commit, to make it easier to review that diff specifically.

• At least two implementers are interested (and none opposed):
  • Firefox has already implemented these methods, but on FileSystemHandle.
  • Chromium has already implemented these methods.
  • WebKit has already implemented the two-argument version of this method, but on FileSystemHandle.
• Tests are written and can be reviewed and commented upon at:
  • https://github.com/web-platform-tests/wpt/blob/master/fs/script-tests/FileSystemFileHandle-move.js
• Implementation bugs are filed:
  • Chromium: N/A
  • Gecko: … (Needs to move the existing methods from FileSystemHandle to FileSystemFileHandle)
  • WebKit: … (Needs to move the existing method from FileSystemHandle to FileSystemFileHandle, and add the other two overload signatures)
• MDN issue is filed: …
• The top of this comment includes a clear commit message to use.

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[annevk]

Thank you for tackling this! I hope @a-sully can review this. I left a couple of editorial nits.

[annevk]

We don't list the types here. This should maybe also use <p> as in the output all these algorithms look squished together.

[valadaptive]

Removed the types from the algorithm headers (hopefully I didn't miss any).

I chose not to use <p> since it currently only appears twice in the entire document, at the footer. If we want to use <p> in more places for clarity, I think it's better to do that consistently as a separate change than to introduce it here.

[annevk]

Initialize this variable in the outer scope once for clarity as you need it later. And then overwrite it for this non-null case. We usually keep "null" without markup btw, not null, just null.

[valadaptive]

Initialized it to "not taken", overwriting it inside the conditional.

I also changed all the uses of null in the new section to plain "null". There are some other pre-existing areas that use null; maybe another editorial change for later.