Feature Request: Per commit modified_files, JSONDiffForFile, fileMatch

**Describe the feature**

Converting https://github.com/danger/danger-js/discussions/1335 to a feature request

**Expected behavior**

There are use-cases where you want to analyse files changed by a commit:
1. you changed package.json but not yarn.lock
2. you pinned a package and didn't leave commit message body why you pinned.
3. modified source, but didn't add/update tests

and those per commit, not whole merge request.

therefore would want to have `commit.modified_files`, `commit.JSONDiffForFile`, `commit.fileMatch` methods:

```ts
const packageDiff = await commit.JSONDiffForFile("package.json")

if (packageDiff.dependencies) {
const newDependencies = packageDiff.dependencies.added;
if (newDependencies.includes(blacklist)) {
fail(`${commit.sha}: Do not add ${blacklist} to our dependencies, see CVE #23`);
}
}

const packageJson = commit.fileMatch("package.json");
const packageLock = commit.fileMatch("yarn.lock");

if (packageJson.modified && !packageLock.modified) {
warn("${commit.sha}: This commit modified `package.json`, but not `yarn.lock`");
}

const testChanges = commit.modified_files.filter(filepath => filepath.match(/(test|cypress)/));
if (testChanges.length < 1) {
warn(`${commit.sha}: This commit does not have any changes in automated tests. Please consider adding some tests.`)
}
```

Start by reviewing discussion #1335 and the usage examples in this issue, then trace how per-commit data is currently exposed. Define the scope and completion criteria for modified_files, JSONDiffForFile, and fileMatch, including their behavior for the package and test-change cases; no files or tests are named in the payload.

Written by the indexing model from the issue text.