chore!: rename value description types to be more descriptive

[hcsch]

• Use plural for enum describing a value descriptions statement
• Use same spelling for value for a single value description
• Use descriptive names for fields of a value description

I stumbled across the ValDescription type while using https://github.com/technocreatives/dbc-codegen, and was a bit confused by the fields being named a and b. I hope this commit can alleviate such confusion for others in the future.

This is a breaking change, so it'll likely have to wait for a future major release.

[nyurik]

the a and b fields have been renamed in #45, released in v7.0.0. I am a bit uncertain if the churn required to rename enum ValueDescription to ValueDescriptions and struct ValDescription -> ValueDescription makes sense tbh... unless more folks chime in on this? Regardless, there is not much sense to keep this PR open as the code has diverged significantly - easier to rename in a new PR