fix: quote SET dp0 in cmd shims to support paths with shell metacharacters

[ashrafiucse]

Fixes #45

What

The .cmd shims generated by cmd-shim set the batch file's directory with an unquoted SET:

SET dp0=%~dp0

When the shim lives in a path containing cmd.exe metacharacters (e.g. C:\test(n&pm)\ or ...\Playwright & API\...), the shell parses the & as a command separator. Two very confusing failures follow:

• 'API\...\node_modules\.bin\' is not recognized as an internal or external command — cmd tries to execute the part of the path after the &
• Cannot find module 'C:\...\Playwright Learn\@playwright\test\cli.js' — dp0 got truncated right before the &, so the target resolves against the wrong base directory

This quotes the assignment using the standard safe idiom, which is already used for _prog in the same file:

SET "dp0=%~dp0"

Notes

• Covers &, ^, ( and ) in paths. A literal % in the path remains unfixable in batch syntax.
• Added a Windows-only functional test that creates a shim inside a dir & name folder and executes it through cmd.exe. Existing tests only snapshot the generated files, which is why a content-level regression like this could go unnoticed.
• Snapshots updated.
• This is a resubmission of fix #45 set command quoted #53, which was closed as stale.