* dialect/sql/schema: add method to create a named versioned migration file
* doc/md: documentation for named versioned migrations
* entc/gen/template/dialect/sql/feature: add NamedDiff method to create named versioned migration files
* all: go generate
* doc/md: apply CR
* dialect/sql/schema: fix go doc
* all: update atlas
* dialect/sql/schema: diff connected DB and defined schema and write changes to migration file
* dialect/sql/schema: use migration directory and formatter directly instead of a planner
* all: update atlas
* cmd: add command to create a new migration file
* entc/gen: generate main.go with migrate diff example
* all: regenerate
* cmd/internal/base: make linter happy
* all: support Go 1.16 in versioned migrations main.go
* entc/gen: put versioned migrations behind feature flag
* all: regenerate
* cmd/ent: driver-prefixed dsn in migrate diff command
* cmd/internal/base: remove prefix from migrate import
* cmd/internal/base: use cobra.CheckErr
* cmd: remove diff command
* entc/gen/template/dialect/sql/feature: remove generated main.go
* all: rebase on master and go mod tidy
* all: regenerate
Also, for some reason, the TimeMixin.UpdateTime was an immutable field,
but this was incorrent, because the codegen just skip generating
update setters to it. Removing the Immutable modifier allows users
to set this field explicitly.
