Add `SemanticVersion.valid?` and `SemanticVersion.parse?`

[devnote-dev]

Currently the only way to "test" a semantic version string is to rescue from SemanticVersion.parse. Such helper methods would be really convenient given that there's no regex or alternative method publicly exposed to do so. It seems like these once existed in an old PR reworking the API and just didn't make it to the existing one, and looking at the implementation it's relatively straightforward to add.