fix: fast refresh is enabled if FAST_REFRESH environment variable is not defined

[martinfrancois]

As pointed out here: #9913 (comment)

The comment in the code clearly states // which is why it's disabled by default. suggesting react-refresh should be disabled by default.
But if the environment variable FAST_REFRESH is not set at all, the condition process.env.FAST_REFRESH !== 'false' evaluates to true (as undefined !== 'false'), enabling react-refresh.

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[n3tr]

@martinfrancois I think Fast Refresh is enabled by default per #9350.

Maybe @iansu forgot to update the comment.

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[martinfrancois]

@n3tr true! In this case, let's change the comment, thanks!

[martinfrancois]

@petetnt thanks for approving! Will this PR be merged at one point?

[stale]

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