[now-next] Add Beta iSSG Support

[ijjk]

Adds support for new SPRv2 handling in Next.js.

x-ref: #3021

[codecov-io]

Codecov Report

Merging #3067 into canary will increase coverage by 1.96%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           canary    #3067      +/-   ##
==========================================
+ Coverage   11.47%   13.43%   +1.96%     
==========================================
  Files         268      268              
  Lines       10096    10320     +224     
  Branches     1313     1222      -91     
==========================================
+ Hits         1159     1387     +228     
- Misses       8847     8871      +24     
+ Partials       90       62      -28

Impacted Files	Coverage Δ
src/util/errors-ts.ts	47.48% <0%> (-0.03%)	⬇️
src/util/dev/router.ts	82.35% <0%> (+3.28%)	⬆️
src/util/get-files.ts	91.66% <0%> (+3.88%)	⬆️
src/util/dev/yarn-installer.ts	90.74% <0%> (+5.02%)	⬆️
src/util/dev/server.ts	64.8% <0%> (+5.56%)	⬆️
src/util/dev/builder.ts	78.32% <0%> (+5.59%)	⬆️
src/util/output/create-output.ts	66.66% <0%> (+7.29%)	⬆️
src/util/dev/builder-cache.ts	84.66% <0%> (+7.39%)	⬆️

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