fix(core): fix renderPageToHtml for work for all next.js versions

[dphang]

• Try to check if html is a string or not (and call latest resultToString if the latter), if it fails then use the page's renderReqToHtml (without renderMode) which should work as a fallback.

[slsnextbot]

Handler Size Report

There are changes to handler sizes. Please review.

Base Handler Sizes (kB) (commit 934675b)

{
    "Lambda@Edge": {
        "Default Lambda": {
            "Standard": 1239,
            "Minified": 440
        },
        "API Lambda": {
            "Standard": 91,
            "Minified": 37
        },
        "Image Lambda": {
            "Standard": 906,
            "Minified": 360
        },
        "Regeneration Lambda": {
            "Standard": 634,
            "Minified": 227
        }
    }
}

New Handler Sizes (kB) (commit 27ffbdc)

{
    "Lambda@Edge": {
        "Default Lambda": {
            "Standard": 1248,
            "Minified": 447
        },
        "API Lambda": {
            "Standard": 99,
            "Minified": 44
        },
        "Image Lambda": {
            "Standard": 914,
            "Minified": 367
        },
        "Regeneration Lambda": {
            "Standard": 642,
            "Minified": 234
        }
    }
}

[codecov]

Codecov Report

Merging #1611 (27ffbdc) into master (214f8a2) will decrease coverage by 0.04%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #1611      +/-   ##
==========================================
- Coverage   84.34%   84.29%   -0.05%     
==========================================
  Files          98       98              
  Lines        3437     3439       +2     
  Branches     1020     1020              
==========================================
  Hits         2899     2899              
- Misses        478      480       +2     
  Partials       60       60              

Impacted Files	Coverage Δ
packages/libs/core/src/utils/renderUtils.ts	80.00% <66.66%> (-20.00%)	⬇️

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