ci: Group Dependabot updates and track GitHub Actions and the Rust toolchain

[mangas]

Reduces Dependabot to one PR per ecosystem per week, and adds two ecosystems it doesn't track today.

• npm: the directory moves from tests/integration-tests to /, since the pnpm workspace root and lockfile are at the repo root. The @graphprotocol/graph-* updates are grouped into one PR.
• cargo: all crate updates are grouped into one PR. wasmtime* keeps its own PR. It ships a major version every month, and some need code changes in runtime/: 2 of the last 6 bumps did (38 → 44 touched 7 files). In the group, one of those would hold back every other crate update. The existing ignore list is unchanged.
• github-actions (new): action updates are grouped into one PR.
• rust-toolchain (new): bumps rust-toolchain.toml.
• All ecosystems: commit messages use the build(deps): ... prefix.