WIP - Switch add-entity command to new php format

[colemanw]

Just some ideas/WIP so far...

[totten]

Good start @colemanw.

I've been playing around with this a bit, and a few things pop out:

• I detect some ambivalence about whether to generate DAO files. I think we should stick to the earlier plan and continue making them. There are too many balls in the air. If we toss in a few more balls, then we'll never finish consolidating. (In the future, we can open #5666: Imagine a world without DAO...)
• We have to rework tests/e2e/AddEntityTest.php -- since the actual substance+flow of the generator is changing.
• AddEntityTest says things like: "Make sure the newly generated entity produces valid SQL schema." That makes sense for E2E test. But this can't be resolved without having (dev/core#4999) Support Entity Framework v2 for extensions #331. On the flipside, IIRC, #331 was hitting some failures because generate:entity wasn't giving the right kind of skeleton (which would be provided by this #341).
• In other words, #331 and #341 are interdependent. They simply touch different aspects of the same project. (Project: Support extension lifecycle with schema/*.php)
• So I'm gonna combine them (i.e. merge 341 into 331).
• A side-effect of PhpData - Use structured document instead of regex #358 is that it should allow simpler fix for the dephell between fn() and CIVIVER=5.57.