feat: embed code context into SARIF output

This is preparation for Fingerprints that are coming in next version of csdiff.

csutils/csdiff#168

> It hashes the data that csdiff uses in its matching algorithm
> and the line content without spaces. For this fingerprint to
> be computed, the results need to include the line content for
> the key event in the format produced by `csgrep --embed-context`.

src/index.sh

@@ -101,6 +101,7 @@ shellcheck_version=$(get_shellcheck_version)
 # GitHub requires an absolute path, so let's remove the './' prefix from it.
 csgrep \
   --strip-path-prefix './' \
+  --embed-context 4 \
   --mode=sarif \
   --set-scan-prop='tool:ShellCheck' \
   --set-scan-prop="tool-version:${shellcheck_version}" \