Reduce size of uploaded crate (#936)

Currently, the compressed size of the `cargo-semver-checks` crate on
crates.io is 1.6MiB, as listed on crates.io and lib.rs. This is mostly
attributed to the brand and test directories, which are not essential
for the crate to compile. We can avoid uploading these directories by
explicitly excluding them in the manifest using `package.exclude`.

Cargo.toml

@@ -10,6 +10,7 @@ readme = "./README.md"
 keywords = ["semver", "linter", "check", "crate", "cargo"]
 categories = ["command-line-utilities", "development-tools::cargo-plugins"]
 rust-version = "1.77"
+exclude = [".github/", "brand/", "scripts/", "test_crates/", "test_outputs/", "tests/"]
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html