Update level-ttl to be compatible with abstract-level

[maxlath]

[Reopening #138 with a different branch name, to be able to push to my main branch without affecting this PR, sorry for the noise]

I started this work without a clear idea of where I was going, mostly to figure-out how to make level-ttl work with rave-level in my use-case, thus the many changes. But maybe that work can be useful for others?

Changes summary

• Replaced callbacks with promises. This required to replace lock with async-lock. I dropped support for callbacks, which apparently will be the way going forward
• Converted from CommonJS to ESM as that seems the most future-proof (but this wouldn't be hard to revert if needed)
• Updated tests with promises and mocha: I went that way because that's the tool I know, but if important for consistency with the other modules, I guess it could be converted back to using tape. For tests coverage, I replaced nyc with c8 as the former doesn't seem to support ESM, but I couldn't find how to produce lcov.info with c8, so that could be a reason to switch back to CJS if that's important
• Added types. I have some uncertainties around the Encoding type, proofreading welcome.