feat: granular --unstable-* flags

[bartlomieju]

This commit adds granular --unstable-* flags:

• "--unstable-broadcast-channel"
• "--unstable-ffi"
• "--unstable-fs"
• "--unstable-http"
• "--unstable-kv"
• "--unstable-net"
• "--unstable-worker-options"
• "--unstable-cron"

These flags are meant to replace a "catch-all" flag - "--unstable", that
gives a binary control whether unstable features are enabled or not. The
downside of this flag that allowing eg. Deno KV API also enables the FFI
API (though the latter is still gated with a permission).

These flags can also be specified in deno.json file under unstable key.

Currently, "--unstable" flag works the same way - I will open a follow up
PR that will print a warning when using "--unstable" and suggest to use
concrete "--unstable-*" flag instead. We plan to phase out "--unstable"
completely in Deno 2.

[lucacasonato]

I am generally in favor of this, but also --unstable-worker is so meaningless! Shouldn't we generally have flags for specific features?

[bartlomieju]

I am generally in favor of this, but also --unstable-worker is so meaningless! Shouldn't we generally have flags for specific features?

There are potentially many unstable features for Worker API (eg. loading an import map), would we really want separate flags for each of these features if they are contained in new Worker() constructor's deno option?

[nayeemrmn]

I think it should be --unstable-worker-options at least.

[bartlomieju]

I think it should be --unstable-worker-options at least.

I'm fine with that name 👍

[dsherret]

LGTM! (once the config file test is added)