[dev] use webpack mode: development

[EverlastingBugstopper]

if target.type == "webpack" && target.webpack_config.is_none(), then set mode: development in webpack config.

This will speed up builds on live reload and improve the experience of using wrangler dev

[xtuc]

also serve the source map locally

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This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

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This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

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This issue has been automatically marked as stale because it has not had recent activity in the last 180 days. It will be closed if no further activity occurs in the next week. Please feel free to comment if you'd like it to remain open, and thank you for your contributions.

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