Add PowerShell installation script scripts/install.ps1 for Windows

[k5602]

Description

Companion to #46 (Unix installation script). Provide a PowerShell installation script scripts/install.ps1 to allow single-command installation on Windows:

irm https://raw.githubusercontent.com/metacall/meta-ast/main/scripts/install.ps1 | iex

Requirements

1. Detect target architecture (x86_64-pc-windows-msvc).
2. Query GitHub Releases API for the latest release or accept an explicit -Version parameter.
3. Download the matching .zip / binary release artifact.
4. Verify the SHA-256 checksum against the published SHA256SUMS.
5. Extract and install meta-ast.exe into ~\.meta-ast\bin (or %LOCALAPPDATA%\meta-ast\bin).
6. Add the installation directory to user PATH if not already present.

Helpful Pointers

• Look at release artifact naming in .github/workflows/ci.yml and GitHub releases.
• Ensure the script runs with default execution policies where possible.