This PR forks a new version of the `zed_extension_api` in preparation
for new changes.

We're jumping from v0.6.0 to v0.8.0 for the WIT because we released
v0.7.0 of the `zed_extension_api` without any WIT changes (it probably
should have been v0.6.1, instead).

Release Notes:

- N/A