Support further ListBlock nesting

[zerolab]

ListBlock in StructBlock in StructBlock in ListBlock (and probably a few further levels down the rabbit hole`

Test build: https://zerolab.org/_dumps/wagtail_localize-1.1.2rc1-py3-none-any.whl

[codecov-commenter]

Codecov Report

Merging #559 (b00a671) into main (eb825ed) will decrease coverage by 0.05%.
The diff coverage is 69.23%.

@@            Coverage Diff             @@
##             main     #559      +/-   ##
==========================================
- Coverage   90.88%   90.82%   -0.06%     
==========================================
  Files          37       37              
  Lines        3303     3315      +12     
  Branches      537      539       +2     
==========================================
+ Hits         3002     3011       +9     
- Misses        176      179       +3     
  Partials      125      125              

Impacted Files	Coverage Δ
wagtail_localize/segments/extract.py	86.92% <55.55%> (-1.35%)	⬇️
wagtail_localize/test/models.py	97.67% <100.00%> (+0.04%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update eb825ed...b00a671. Read the comment docs.

[chris48s]

I'm assuming the netlify build failures are unrelated to these functional changes?