fix(core): fix renderReqToHtml function for next.js 12

[dphang]

Seems that the proper way to transform HTML to string is using toUnchunkedString() in Next.js 12. So we should try that first if the function exists on the page and otherwise fallback to other methods.

[slsnextbot]

Handler Size Report

There are changes to handler sizes. Please review.

Base Handler Sizes (kB) (commit e7c81ef)

{
    "Lambda": {
        "Default Lambda": {
            "Standard": 1495,
            "Minified": 658
        },
        "Image Lambda": {
            "Standard": 1461,
            "Minified": 792
        }
    },
    "Lambda@Edge": {
        "Default Lambda": {
            "Standard": 1506,
            "Minified": 664
        },
        "Default Lambda V2": {
            "Standard": 1498,
            "Minified": 660
        },
        "API Lambda": {
            "Standard": 632,
            "Minified": 318
        },
        "Image Lambda": {
            "Standard": 1470,
            "Minified": 797
        },
        "Regeneration Lambda": {
            "Standard": 1163,
            "Minified": 537
        },
        "Regeneration Lambda V2": {
            "Standard": 1228,
            "Minified": 564
        }
    }
}

New Handler Sizes (kB) (commit 4924d0e)

{
    "Lambda": {
        "Default Lambda": {
            "Standard": 1495,
            "Minified": 658
        },
        "Image Lambda": {
            "Standard": 1461,
            "Minified": 792
        }
    },
    "Lambda@Edge": {
        "Default Lambda": {
            "Standard": 1506,
            "Minified": 664
        },
        "Default Lambda V2": {
            "Standard": 1498,
            "Minified": 660
        },
        "API Lambda": {
            "Standard": 632,
            "Minified": 318
        },
        "Image Lambda": {
            "Standard": 1470,
            "Minified": 797
        },
        "Regeneration Lambda": {
            "Standard": 1162,
            "Minified": 537
        },
        "Regeneration Lambda V2": {
            "Standard": 1228,
            "Minified": 564
        }
    }
}

[codecov]

Codecov Report

Merging #1971 (4924d0e) into master (e7c81ef) will decrease coverage by 0.06%.
The diff coverage is 54.54%.

@@            Coverage Diff             @@
##           master    #1971      +/-   ##
==========================================
- Coverage   83.79%   83.72%   -0.07%     
==========================================
  Files         100      100              
  Lines        3609     3612       +3     
  Branches     1140     1141       +1     
==========================================
  Hits         3024     3024              
- Misses        525      528       +3     
  Partials       60       60              

Impacted Files	Coverage Δ
packages/libs/core/src/utils/renderUtils.ts	70.58% <54.54%> (-15.13%)	⬇️

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