`deno bundle`: Add `--keep-names` flag

[egfx-notifications]

We used deno bundle in its original implementation, then with its removal migrated to a custom build script with esbuild

Now that deno bundle is back we wanted to migrate back to using it, but one thing we are missing right now to recreate our current esbuild config is the keepNames option.

I tried adding a --keep-names flag to deno bundle and this does what it should do, but before submitting a PR I would like to discuss the best way forward.

1. Would you appreciate a PR adding the --keep-names flag to deno bundle or is this a requirement too specific to be added as its own flag?
2. If you agree to have this added, should we just add the --keep-names flag to deno bundle and use EsbuildFlagsBuilder.raw_flag() internally? Or should we add EsbuildFlagsBuilder.keep_names() to https://github.com/denoland/esbuild_client/blob/main/src/flags.rs ?
3. Considering that there are more valid esbuild flags currently not supported by deno bundle, should we add a --raw-esbuild-options flag to deno bundle instead of adding individual esbuild flags everytime someone comes up with the requirement?