Add `lightningcss` and `minify-js` feature flags

This PR adds the lightningcss and minify-js feature flags to the minify-html crate.

It allows significantly lighter and faster compilation for users who don't use the provided CSS/JS minifiers. It may also help solving dependency hell issues.

Changes:

• In minify-html/Cargo.toml, dependencies lightningcss and minify-js are optional.
• Feature flags lightningcss and minify-js are enabled by default, therefore the current behavior is preserved.
• In Cfg, fields minify_css and minify_js are behind feature flags.
• Tests that use the CSS/JS minifiers are behind feature flags.

In cargo release mode of the binary size used minify-html.

• Those two dependencies about 10MB.
• And minify-html-common about 9M