Support `alias` in stub mode

[peterroe]

Describe the feature

export default defineBuildConfig({
    entries: [ './packages/core/jimu' ],
    failOnWarn: false,
    stub: true,
    alias: {		// Not effective when stub is true
        '@': resolve(__dirname, './')
    },
});

Additional information

• [x] Would you be willing to help implement this feature?