[now-build-utils] Add types for SPRv2

[leo]

This pull request adds two new types in preparation for Serverless Prerendering v2.

The first one is Prerender, which represents a pre-rendering bundle:

interface Prerender {
  expiration: number;
  lambda: Lambda;
  fallback: FileBlob | FileFsRef | FileRef;
}

The second one is PrerenderGroup, which defines a list of Prerender that should be invalidated at the same time – if one of them is invalidated:

interface PrerenderGroup {
  items: Array<Prerender>;
}

[codecov-io]

Codecov Report

Merging #3021 into canary will increase coverage by 2.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           canary    #3021      +/-   ##
==========================================
+ Coverage   11.33%   13.36%   +2.03%     
==========================================
  Files         267      267              
  Lines       10032    10273     +241     
  Branches     1308     1216      -92     
==========================================
+ Hits         1137     1373     +236     
- Misses       8803     8838      +35     
+ Partials       92       62      -30

Impacted Files	Coverage Δ
src/util/dev/builder-cache.ts	80.74% <0%> (+3.47%)	⬆️
src/util/get-files.ts	91.66% <0%> (+3.88%)	⬆️
src/util/dev/router.ts	82.69% <0%> (+4.12%)	⬆️
src/util/dev/builder.ts	77.41% <0%> (+4.34%)	⬆️
src/util/dev/yarn-installer.ts	90.74% <0%> (+5.02%)	⬆️
src/util/dev/server.ts	66.43% <0%> (+5.73%)	⬆️
src/util/output/create-output.ts	43.58% <0%> (+12.33%)	⬆️

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