This commit adds `--unstable-hmr` flag, that enabled Hot Module Replacement.
This flag works like `--watch` and accepts the same arguments. If
HMR is not possible the process will be restarted instead.
Currently HMR is only supported in `deno run` subcommand.
Upon HMR a `CustomEvent("hmr")` will be dispatched that contains
information which file was changed in its `details` property.
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Co-authored-by: Valentin Anger <syrupthinker@gryphno.de>
Co-authored-by: David Sherret <dsherret@gmail.com>
