Facilitate users switching UAs

[jyasskin]

https://w3ctag.github.io/user-agents/#ecosystem talks about it being possible for users to switch UAs, but perhaps we should point out that it's user-hostile for a UA to do things that make it hard for users to switch? For example, it's good that it's easy to switch from Chrome to Firefox, and Chrome would be violating its duties to its users if it intentionally obfuscated the bookmarks and history to make it hard for Firefox to copy them.

There might be tension here with the goodness of UAs innovating: any innovation won't immediately be copyable to other UAs, and the formats involved in a new innovation might change too quickly for migration scripts to keep up, and we don't want to say that's harmful in itself.