Remove hbs dependency

[Naktibalda]

Reasons:

• hbs view engine is registered, but not used anywhere.
• handlebars library had 2 security vulnerabilies recently, but hbs is slow to update,

We discussed it in Slack last week:

benbrown 8:49 PM
It’s in the conversation system

naktibalda 10:03 AM
https://github.com/howdyai/botkit/blob/v4.6/packages/botkit/src/conversation.ts#L858 uses mustache, not handlebars
packages/botkit/src/conversation.ts:858

        outgoing.text = mustache.render(outgoing.text, { vars: vars });

naktibalda 10:12 AM
The only package using hbs templates is docs, https://github.com/howdyai/botkit/tree/master/packages/docs
naktibalda 10:19 AM
hbs isn't used by botkit, but it could be used by third-party plugins so removing it is a BC-break and requires major version bump. Not worth it. (edited)
benbrown 2:39 PM
Ah I think it would be safe
Plugins need to declare their own deps (edited)

[Naktibalda]

hbs: 4.1.0 was released today,
it is necessary to either remove it from dependencies or update version constraint to allow upgrade to 4.1.0.

[Naktibalda]

My previous comment was incorrect, there is no need to update version constraint to get a new version of hbs.
It would still be useful to remove this dependency.

[benbrown]

I'm planning on building a new release as soon as #1894 is fixed.