Inconsistency when parsing numbers with underscores

[fh-yuxiao-zeng]

Describe the bug
YAML 1.1 allows to have underscore in number representations, like 123_456 (int) and 123_456.78 (float).

However, this seems partially supported in YamlDotNet.

Underscore is supported when a tag (!!int or !!float) is specified. But when AttemptingUnquotedStringTypeDeserialization is turned on while the tag is missing, the type is not properly inferred.

To Reproduce
Please check this fiddle: https://dotnetfiddle.net/uL0fls

I feel the regular expression here is not sufficient: https://github.com/aaubry/YamlDotNet/blob/master/YamlDotNet/Serialization/NodeDeserializers/ScalarNodeDeserializer.cs#L356

[EdwardCooke]

We tried making that method as optimized as possible. There's a lot of additional checks and it be quite time consuming. The usage of _ vs , seems to me to be very minimal in use so I didn't do anything about them when I built it. If you want to submit a PR changing that we'd probably accept it. Though it might be some time before I can spend on merging and cutting a release.