Add proposed charter for a performance task force

[tidoust]

This proposed charter merges and reformulates texts proposed by @chiefcll in #86 and #88, following a similar structure to previously created task forces (e.g. Media Timed Events TF).

In particular:

• It adds a "mission" section that lists the tasks that the task force would be responsible for.
• It adds a "success criteria" section that lists deliverables.

The text also include an "Out-of-scope" section to note that the TF will not create a benchmark of existing consumer products.

The text uses the term "consumer products" instead of "TV" or "embedded device". I don't feel strongly about the term but note that this is what the Web Media API document uses as well.

The creation of a toolkit to measure performance of web UIs on consumer products could be further refined (also see comment from @jpiesing in https://github.com/w3c/media-and-entertainment/pull/88#issuecomment-1195108292). I'm not clear what the group is willing to do on that front so left that as-is.

[chrisn]

Thanks @tidoust. We discussed on the chairs call today. As the scope is broader than performance, I suggest changing the TF name ("Application Development for Consumer Devices TF"?), and as we want to prioritise developer experience initially, I suggest re-ordering the bullets. I can work on an update to this PR.