feat: add original message formatting helpers with tests

[fluffur]

Example

msg := &botapi.Message{
	Text: "Hello world",
	Entities: []botapi.MessageEntity{
		{
			Type:   botapi.EntityBold,
			Offset: 6,
			Length: 5,
		},
	},
}

fmt.Println(msg.OriginalMD())    // Hello *world*
fmt.Println(msg.OriginalMDV2())  // Hello *world*
fmt.Println(msg.OriginalHTML())  // Hello <b>world</b>

[codecov]

Codecov Report

❌ Patch coverage is 66.82927% with 68 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.63%. Comparing base (ec97c6a) to head (f1e6929).

Files with missing lines	Patch %	Lines
formatting.go	66.82%	57 Missing and 11 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #461      +/-   ##
==========================================
- Coverage   78.89%   78.63%   -0.27%     
==========================================
  Files         118      119       +1     
  Lines        8359     8564     +205     
==========================================
+ Hits         6595     6734     +139     
- Misses       1237     1293      +56     
- Partials      527      537      +10     

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