change msgtype to m.emote to be more standard compliant

[dozro]

Description

Change the msgtype from the custom fyi.cisnt.headpat to the standard m.emote while still keeping the old rendering as fallback for older messages sent from sable.

it will now generate events like the following:

{
  "content": {
    "body": "pats @rye:itsrye.dev",
    "fyi.cisnt.headpat": true,
    "m.mentions": {
      "user_ids": [
        "@rye:itsrye.dev"
      ]
    },
    "msgtype": "m.emote"
  },
  "event_id": "$OOWvyc9_vGKQvv9X_yI-L6Al7cZ88c45THHDP_BuaNg",
  "origin_server_ts": 1773156785613,
  "sender": "@client-testing:itsrye.dev",
  "type": "m.room.message",
  "unsigned": {
    "age": 19,
    "transaction_id": "m1773156785357.1"
  },
  "room_id": "!OVi0VXH2pr4UWMdQvm:itsrye.dev"
}

while keeping the current behaviour and display

fixes #142
@tulir is welcome to look over it, as they raised the issue to me

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings