fix: no underscores after numbers when converting to snake case

[YodaEmbedding]

Summary

Fixes #657

Usage:

>>> betterproto.casing.snake_case("C2S2C c2s2c")
'c2s2c_c2s2c'

>>> betterproto.casing.snake_case("Client2Server CLIENT2SERVER client2Server")
'client2server_client2server_client2server'

>>> betterproto.casing.snake_case("clientToServer")
'client_to_server'

Checklist

• [x] If code changes were made then they have been tested.
  • [x] I have updated the documentation to reflect the changes.
• [x] This PR fixes an issue.
• [ ] This PR adds something new (e.g. new method or parameters).
  • [ ] This change has an associated test.
• [ ] This PR is a breaking change (e.g. methods or parameters removed/renamed)
• [ ] This PR is not a code change (e.g. documentation, README, ...)