lab: Section to fields migration steps

[bastianallgeier]

Description

Don't review yet. This is only for me as a reference for the required steps

[afbora]

@bastianallgeier Tested this quite thoroughly — all section types convert correctly, plugin sections still work through the adapter, layouts (tabs, columns, mixed) come out right, name clashes fail loudly, and I couldn't find any test regressions. Really nice work 👍

One thing I did find: two fields sections with reference lists work on v6/section-to-fields but break here.

title: Reference lists collide

fields:
  one:
    label: One
    type: text
  two:
    label: Two
    type: toggle

sections:
  first:
    type: fields
    fields:
      - one
  second:
    type: fields
    fields:
      - two

Both fields disappear and you get The field "0" already exists in your blueprint.