fix(css): initialize lightningCSS targets when not using options

[ArnaudBarre]

This was a weird behaviour where:

css: {
  transformer: "lightningcss",
},

was not the same as

css: {
   transformer: "lightningcss",
   lightningcss: {},
},

Because without any targets LightningCSS will default to latest specs and do a lot of syntax shortening and vendor-prefixing cleanup, this should always be set to avoid sending non supported CSS to the browser.

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