Default to auto-detected 64-bit MSBuild on 64-bit hosts (breaking change, v4)

Fixes #88

- msbuild-architecture no longer hardcodes a default of "x86". When the
  input is left unspecified, the action now auto-detects: it prefers x64
  when running on a 64-bit machine and the resolved VS/MSBuild install is
  17.0+ (VS 2022+), and falls back to x86 otherwise. An explicit
  msbuild-architecture value is always respected.
- Extracted resolveMSBuildArchitecture()/parseMajorVersion() as pure,
  exported helpers and added jest tests covering the resolution matrix.
- Bumped package.json version to 4.0.0 (breaking change/new major version).
- Updated action.yml input description and README (usage examples now
  reference @v4, new "Breaking Changes in v4" section, and the
  architecture-selection docs rewritten to describe auto-detection).
- Rebuilt dist/index.js via ncc.

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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "setup-msbuild",
-  "version": "3.0.0",
+  "version": "4.0.0",
   "private": true,
   "description": "Helps set up specific MSBuild tool into PATH for later usage.",
   "main": "lib/main.js",