`".e0".parse()` returns `Ok(0.0)`

[lifthrasiir]

Yes, another incarnation of #30344. +.e0 and -.e0 also have this issue. Note that ".".parse() is an error and has a test for it.

cc #32243

[hanna-kruppe]

Does this also date back to 1.0, like #30344 did?

[lifthrasiir]

@rkruppe Yes. The fix would be technically a breaking change.