[now-next] Add loading of custom routes

[ijjk]

This adds loading of redirects and rewrites from the routes-manifest in supported Next.js versions

[codecov-io]

Codecov Report

Merging #3279 into canary will increase coverage by 2.1%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           canary    #3279     +/-   ##
=========================================
+ Coverage    11.8%   13.91%   +2.1%     
=========================================
  Files         267      267             
  Lines       10035    10297    +262     
  Branches     1330     1241     -89     
=========================================
+ Hits         1185     1433    +248     
- Misses       8757     8798     +41     
+ Partials       93       66     -27

Impacted Files	Coverage Δ
src/util/metrics.ts	68.18% <0%> (-2.41%)	⬇️
src/util/dev/builder-cache.ts	85.18% <0%> (+2.57%)	⬆️
src/util/projects/get-project-by-id-or-name.ts	28.57% <0%> (+3.57%)	⬆️
src/util/get-files.ts	91.66% <0%> (+3.88%)	⬆️
src/util/dev/yarn-installer.ts	90.74% <0%> (+5.02%)	⬆️
src/util/prefer-v2-deployment.ts	61.9% <0%> (+5.65%)	⬆️
src/util/dev/server.ts	64.5% <0%> (+5.81%)	⬆️
src/util/dev/builder.ts	71.59% <0%> (+7.25%)	⬆️
src/util/output/create-output.ts	66.66% <0%> (+7.29%)	⬆️

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