Detect leading zero on positive float

[moorereason]

Describe the bug
Given a float such as 0=+0_0.0, go-toml accepts the input as valid.

$ echo -n "0=+0_0.0" | gotoml-test-decoder
{
  "0": {
    "type": "float",
    "value": "0"
  }
}

To Reproduce
https://go.dev/play/p/EDKOatLs5WA

toml.Unmarshal([]byte("0=+0_0.0"), &v)

Expected behavior
Expected to see an error about leading zero on a float number.

Versions

• go-toml: v2 e83cf53
• go version go1.17.5 linux/amd64

Additional context
Found while doing differential fuzzing against tomlplusplus/v3.

[pelletier]

Thanks!