feat(lambda-at-edge): detect and copy next-i18next files

[dphang]

Should fix issues like #1444

This allows for better next-i18next support by copying commonly required files to the default lambda. Also creates a simple framework for adding additional integrations.

[slsnextbot]

Handler Size Report

No changes to handler sizes.

Base Handler Sizes (kB) (commit a5d74d2)

{
    "Lambda@Edge": {
        "Default Lambda": {
            "Standard": 1226,
            "Minified": 432
        },
        "API Lambda": {
            "Standard": 86,
            "Minified": 34
        },
        "Image Lambda": {
            "Standard": 894,
            "Minified": 354
        },
        "Regeneration Lambda": {
            "Standard": 620,
            "Minified": 220
        }
    }
}

New Handler Sizes (kB) (commit d2ac9d7)

{
    "Lambda@Edge": {
        "Default Lambda": {
            "Standard": 1226,
            "Minified": 432
        },
        "API Lambda": {
            "Standard": 86,
            "Minified": 34
        },
        "Image Lambda": {
            "Standard": 894,
            "Minified": 354
        },
        "Regeneration Lambda": {
            "Standard": 620,
            "Minified": 220
        }
    }
}

[codecov]

Codecov Report

Merging #1472 (f8579f2) into master (a5d74d2) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1472      +/-   ##
==========================================
+ Coverage   84.15%   84.25%   +0.09%     
==========================================
  Files          96       98       +2     
  Lines        3371     3392      +21     
  Branches     1006     1010       +4     
==========================================
+ Hits         2837     2858      +21     
  Misses        474      474              
  Partials       60       60              

Impacted Files	Coverage Δ
packages/libs/lambda-at-edge/src/build.ts	97.11% <100.00%> (+0.02%)	⬆️
...-at-edge/src/build/third-party/integration-base.ts	100.00% <100.00%> (ø)
...mbda-at-edge/src/build/third-party/next-i18next.ts	100.00% <100.00%> (ø)

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