feat(core, nextjs-component): improve routing and support cross rewrites for consolidated API pages in default lambda

[dphang]

• experimental: set build.separateApiLambda = false to enable single lambda for pages + APIs
• moves API page routing/rendering to default handler routing, which is cleaner as it already provides abstractions over different route types. It's easier also to add logic in page manifest parsing to treat API pages as another page type, so it lives with the regular HTML/SSG/SSR pages. We can get rewrite logic mostly for free.
• cross rewrites like /page <-> /api/page should now work (there is a simple e2e test added for the basic cases).
• we will also get nice 404 pages instead of the previous text-only, uncustomizable 404 page for APIs.
• this is mainly only tested in e2e next-app-experimental. But if this becomes the default later, we will need to update unit tests

Fixes: #939

[slsnextbot]

Handler Size Report

There are changes to handler sizes. Please review.

Base Handler Sizes (kB) (commit c5859c7)

{
    "Lambda@Edge": {
        "Default Lambda": {
            "Standard": 1271,
            "Minified": 453
        },
        "API Lambda": {
            "Standard": 100,
            "Minified": 44
        },
        "Image Lambda": {
            "Standard": 933,
            "Minified": 371
        },
        "Regeneration Lambda": {
            "Standard": 655,
            "Minified": 237
        }
    }
}

New Handler Sizes (kB) (commit 4a651f5)

{
    "Lambda@Edge": {
        "Default Lambda": {
            "Standard": 1268,
            "Minified": 452
        },
        "API Lambda": {
            "Standard": 101,
            "Minified": 44
        },
        "Image Lambda": {
            "Standard": 933,
            "Minified": 371
        },
        "Regeneration Lambda": {
            "Standard": 655,
            "Minified": 237
        }
    }
}

[codecov]

Codecov Report

Merging #1693 (4a651f5) into master (c5859c7) will increase coverage by 0.08%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master    #1693      +/-   ##
==========================================
+ Coverage   83.86%   83.95%   +0.08%     
==========================================
  Files          98       98              
  Lines        3477     3477              
  Branches     1045     1045              
==========================================
+ Hits         2916     2919       +3     
+ Misses        499      496       -3     
  Partials       62       62              

Impacted Files	Coverage Δ
packages/libs/lambda-at-edge/src/build.ts	96.28% <ø> (ø)
...ackages/libs/lambda-at-edge/src/default-handler.ts	88.88% <ø> (+2.85%)	⬆️
packages/libs/core/src/handle/default.ts	85.71% <33.33%> (-10.84%)	⬇️
packages/libs/core/src/route/page.ts	95.65% <71.42%> (-4.35%)	⬇️
packages/libs/core/src/build/index.ts	95.45% <100.00%> (+0.16%)	⬆️
packages/libs/core/src/build/ssr.ts	100.00% <100.00%> (ø)
...rless-components/nextjs-component/src/component.ts	86.28% <100.00%> (+0.82%)	⬆️

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