IrcParser: make public and speed up by another <=15%

[neon-sunset]

Supersedes #217

Description

We can now make IrcParser public and it will do the right thing (tm) when utilized from user code.

• Add missing doc comments
• Add null/short string guard
• Improve parser perf. by another up to 15% depending on runtime by improving SplitFirst/Last logic (adopting learnings from working on another project)

To be honest, it is pretty meaningless to continue improving parsing performance since TwitchLib impl. is now faster than popular alternatives in Java, Rust and Go (some of them kind of shoot themselves in the foot though). But hey, neuron activation.

Numbers

(before version supposedly roll-forwarded to a newer compiler version too so runtime perf. differences should not be significant)
Before:

BenchmarkDotNet=v0.13.5, OS=macOS 14.0 [Darwin 23.0.0]
Apple M1 Pro, 1 CPU, 8 logical and 8 physical cores
.NET SDK=8.0.100-preview.6.23316.13
  [Host]     : .NET 7.0.1 (7.0.122.56804), Arm64 RyuJIT AdvSIMD
  DefaultJob : .NET 7.0.1 (7.0.122.56804), Arm64 RyuJIT AdvSIMD

Method	Message	Mean	Error	StdDev	Gen0	Gen1	Allocated
Parse	:blub(...) list [66]	76.47 ns	0.191 ns	0.178 ns	0.0587	-	368 B
Parse	:jtv(...)ers. [100]	76.21 ns	0.227 ns	0.201 ns	0.0573	-	360 B
Parse	@bad(...)senE [884]	856.95 ns	2.589 ns	2.295 ns	0.5836	0.0048	3664 B
Parse	@bad(...)Guys [382]	739.43 ns	1.645 ns	1.458 ns	0.4101	0.0010	2576 B
Parse	@msg-(...)live. [74]	122.61 ns	0.380 ns	0.355 ns	0.0994	-	624 B
Parse	@msg(...)ces. [110]	127.62 ns	0.280 ns	0.262 ns	0.1109	-	696 B

After:

BenchmarkDotNet v0.13.7, macOS Sonoma 14.0 (23A5301h) [Darwin 23.0.0]
Apple M1 Pro, 1 CPU, 8 logical and 8 physical cores
.NET SDK 8.0.100-rc.1.23404.1
  [Host]     : .NET 8.0.0 (8.0.23.38103), Arm64 RyuJIT AdvSIMD
  DefaultJob : .NET 8.0.0 (8.0.23.38103), Arm64 RyuJIT AdvSIMD

Method	Message	Mean	Error	StdDev	Gen0	Gen1	Allocated
Parse	:blub(...) list [66]	75.10 ns	0.233 ns	0.194 ns	0.0714	-	448 B
Parse	:jtv(...)ers. [100]	73.24 ns	0.280 ns	0.262 ns	0.0701	-	440 B
Parse	@bad(...)senE [884]	696.84 ns	1.085 ns	0.962 ns	0.5836	0.0038	3664 B
Parse	@bad(...)Guys [382]	586.12 ns	1.104 ns	0.922 ns	0.4101	0.0010	2576 B
Parse	@msg-(...)live. [74]	106.09 ns	0.175 ns	0.155 ns	0.0994	0.0001	624 B
Parse	@msg(...)ces. [110]	109.15 ns	0.183 ns	0.163 ns	0.1109	-	696 B