Improving Buble maintainability and v1.0

[mourner]

Continuing discussion in #220 and #185, I think the project reached a boiling point where we have to break compatibility to a minor extent to be able to land fixes (see recent PRs which are blocked by old Node — #240, #237, #236) and upgrade dependencies, so I propose making it ready for a v1.0.0 release by:

• Dropping Node 4–8 compatibility and committing to only supporting versions of Node that did not reach End of Life (according to the status here).
• Upgrading all dependencies to latest semver releases (including acorn) and keeping them up-to-date from now on.
• Adding release automation with GitHub actions.
• Extending maintainer pool, including @luiscubal and @vitorveiga who made some great contributions recently.

cc @Rich-Harris @adrianheine @nathancahill

[Rich-Harris]

👍 from me on all points