napi/minifier: `compress.unused` option cannot be set correctly from napi binding

[sushichan044]

Description

The compress.unused option implemented in PR #11796 cannot accept boolean values from the napi,
making it impossible to properly control unused variable removal behavior.

Current Behavior

• TypeScript definition declares unused?: true | false | 'keep_assign'
• Rust implementation uses Option<String>, which cannot handle boolean values correctly
• Setting unused: true or unused: false doesn't work as expected and causes panic

Expected Behavior

The option should accept three types of values:

• unused: true → Remove unused variables (CompressOptionsUnused::Remove)
• unused: false → Keep unused variables (CompressOptionsUnused::Keep)
• unused: 'keep_assign' → Keep variables with assignments (CompressOptionsUnused::KeepAssign)

Related

• PR feat(minifier): remove unused variable declaration #11796 - feat(minifier): remove unused variable declaration