Should we have a setting that is the opposite of `LEGACY_VM_SUPPORT`

i.e. a way to say "I want to target only the current latest browsers".

Maybe there is some way to do this today, but I don't know what it is.

Setting `-sMIN_CHROME_VERSION=-1`, etc, doesn't quite work because that says we don't want support chrome at all.

Perhaps the explicit `-sLEGACY_VM_SUPPORT=0` could set all of the MIN versions to some huge value? Or is that too magical?

Perhaps a new setting list `-sBLEEDING_EDGE_VM_SUPPORT_ONLY`.. i can't think if a good/short name for that though.

Start by locating the configuration handling for LEGACY_VM_SUPPORT and the MIN_CHROME_VERSION and related settings. Compare the current behavior when LEGACY_VM_SUPPORT is disabled with the requested current-browser-only behavior, then establish the setting name and exact version semantics before implementation; the issue names no test or file.

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