Update File System Access (harmful)

[annevk]

Closes #154 (again).

[tantek]

I'd drop "large" (s/large subset/subset). Given tendency of our positions to be overly optimistically interpreted by advocates / other implementers, we need to err on the side of being more critical of the harms (which this PR helps a lot with), and reserved about the things we're ok with or like.

Review addressed

[jimmywarting]

Could I ask if #154 maybe be unlocked again, please?
It can remain closed. But i hope it have cooled down enough over a couple of years that it isn't longer overwhelmed and some have moved their focus elsewhere.

it would be sad if FS access never gets to see the light of day.
So i hope ppl can remain to the topic and discuss ways to:

• Making it safe(er). and what Mozilla/Firefox could do to implement it (maybe just parts of it)
• Discuss what "FS access API" can do to make Mozilla reconsider.

Maybe edit the original message with a footer describing what is okey to post and what's not.
Maybe put in what it's about the "FS access" that is considered harmful.