[Windows] Missing features for the new msi installer

PR #200734 changed old NSIS installer to WIX msi installer, two features are missing from new msi installer, I think they useful for third party tools to locate installed LLVM.
1. Add LLVM to system or user `PATH` environment variable.

2. Add LLVM installation path to registry.
```reg
Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\LLVM\LLVM]
@="C:\\Program Files\\LLVM\\"
```

It may be useful to record LLVM version in registry (e.g. `ClangCl` toolchain use version to locate include path).

cc @tstellar, @zmodem, @boomanaiden154

Start by reviewing PR #200734 and the current Windows MSI installer changes to identify where installer properties, environment variables, and registry entries are defined. Verify the implementation by installing the MSI and checking PATH behavior and the requested LLVM registry key, including whether the version is recorded.

Written by the indexing model from the issue text.