fix(dev): add postinstall script to configure git symlinks on Windows

[mynameistito]

Summary

• Add script/windows-symlinks.ts that configures git symlinks on Windows
• Add postinstall script to automatically run after bun install
• Fixes TypeScript errors in symlinked .d.ts files on Windows

Problem

On Windows, git config core.symlinks defaults to false, causing git to write symlinks as plain text files. TypeScript then reads the symlink path string as code, resulting in errors like:

src/custom-elements.d.ts(1,1): error TS1128: Declaration or statement expected.

Solution

The postinstall script:

1. Detects if running on Windows (exits early on other platforms)
2. Enables core.symlinks in git config
3. Restores the affected symlinked files from git index

Requirements

• Windows 10+ with Developer Mode enabled, OR
• Run terminal as Administrator

Closes #13397

[mynameistito]

as discussed in discord, should not be changing git configs.