Read config from `.config/mutants.toml` and `[workspace.metadata.mutants]`

[theaddonn]

cargo-mutants already reads .cargo/mutants.toml, thanks! I'm trying to get Rust tools to agree on where config can live, and opening similar requests with rustfmt, clippy, nextest and cargo-deny. Today a typical repo has:

.cargo/config.toml     # cargo
.cargo/mutants.toml    # cargo-mutants (default location)
.cargo/deny.toml       # cargo-deny (supported alongside deny.toml)
.config/nextest.toml   # cargo-nextest (only location)
rustfmt.toml           # rustfmt
clippy.toml            # clippy

Two additions would let cargo-mutants fit whichever layout a project picks:

1. Also read .config/mutants.toml, checked after .cargo/mutants.toml so the current default keeps priority. nextest only reads .config/, so projects that consolidate there currently need --config for mutants.
2. Also read [workspace.metadata.mutants] (or [package.metadata.mutants] for a single-crate root) from the root Cargo.toml, as the lowest-priority fallback when no config file exists. Several tools (e.g. bacon, typos) already support this, and it lets small projects avoid a separate file entirely.

--config and --no-config would behave as they do now.