First bits of the Context overhaul.

Reintroduces separate contexts for users, channels, connections (now split into sessions and connections) and user-channel associations.
It builds which is as much assurance as I can give about the stability of this commit, but its also the bare minimum of what i like to commit sooooo
A lot of things still need to be broadcast through events throughout the application in order to keep states consistent but we'll cross that bridge when we get to it.
I really need to stop using that phrase thingy, I'm overusing it.

SharpChatCommon/StringDiff.cs

@@ -0,0 +1,13 @@
+namespace SharpChat;
+
+public readonly struct StringDiff(
+    string before,
+    string? after,
+    StringComparison comparisonType = StringComparison.Ordinal
+) : Diff {
+    public readonly string Before = before ?? throw new ArgumentNullException(nameof(before));
+    public readonly string After = after ?? before;
+    public readonly StringComparison ComparisonType = comparisonType;
+
+    public bool Changed => !Before.Equals(After, ComparisonType);
+}