isEnvDevelopment enable sourcemaps

[saidmoya12]

sourceMap: isEnvProduction ? shouldUseSourceMap : isEnvDevelopment, (Enable sourcemaps in dev)
added stylus-loader

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[bugzpodder]

We had several issues with enabling sourcemaps in dev, including FOUC (#6399) and slowness (#6508).

[stale]

This pull request has been automatically marked as stale because it has not had any recent activity. It will be closed in 5 days if no further activity occurs.

[stale]

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