Add wasmtime pattern to Dependabot

[jeffcharles]

Description of the change

Have Dependabot group wasmtime updates together.

Why am I making this change?

Wasmtime updates won't succeed unless they're all updated together.

Checklist

• I've updated the default plugin import namespace and incremented the major version of javy-plugin-api if the QuickJS bytecode has changed.
• I've updated the relevant CHANGELOG files if necessary. Changes to javy-cli, javy-plugin, and javy-plugin-processing do not require updating CHANGELOG files.
• I've updated the relevant crate versions if necessary. Versioning policy for library crates
• I've updated documentation including crate documentation if necessary.