Register a problem matcher for MSBuild/csc errors and warnings

- Add .github/msbuild.json with two owners: one matching file-based
  diagnostics (File(line,col): error/warning CODE: msg [project]) and
  one matching engine-level errors without file/line info.
- Register the matcher in src/main.ts via the ##[add-matcher] workflow
  command, resolved relative to __dirname so it works from the ncc
  bundle in dist/.
- Add src/__tests__/msbuild-matcher.test.ts validating the matcher
  regexes against representative CS/MSB error and warning lines.
- Document the new problem matcher in README.md.
- Rebuild dist/index.js via npm run build && npm run pack.

Fixes #4

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

dist/index.js

@@ -42,9 +42,16 @@ if (VS_VERSION !== 'latest') {
     VSWHERE_EXEC += `-version "${VS_VERSION}" `;
 }
 core.debug(`Execution arguments: ${VSWHERE_EXEC}`);
+function registerProblemMatcher() {
+    const matcherPath = path.join(__dirname, '..', '.github', 'msbuild.json');
+    core.info(`##[add-matcher]${matcherPath}`);
+}
 function run() {
     return __awaiter(this, void 0, void 0, function* () {
         try {
+            // register the problem matcher so MSBuild/csc errors and warnings are
+            // annotated in the workflow log and any related pull request
+            registerProblemMatcher();
             // exit if non Windows runner
             if (IS_WINDOWS === false) {
                 core.setFailed('setup-msbuild can only be run on Windows runners');