Add workaround for Math.pow constant folding (only for JS builds)

[MaxGraey]

Many browsers have fast paths for Math.pow if exponents is integer-like. But it leads to imprecisions like 10 ** 208 != 1e208 or 10 ** -5 != 1e-5. For avoid this in our constant folding special case I add some clever workaround. Also add tests.

For more info see this comment: https://bugzilla.mozilla.org/show_bug.cgi?id=1775254#c13

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[dcodeIO]

Wdyt of moving the "accurate pow" to src/util/math as a helper? Seems good to do just to separate the relevant documentation out of the compiler itself :)