Update VSInternalContinueCharacter* serialization (#80085)

Exception counts during the RazorEditingTEsts.CompletionInLSP test is off the chart due to JS completion utilizing VSInternalContinueCharacter classes. These classes were being improperly serialized. The vslanguageserverclient part of this change went in via https://devdiv.visualstudio.com/DevDiv/_git/VSLanguageServerClient/pullrequest/665374 and has made it's way to the latest Int.Main.

This PR simply does the same thing as the vslnaguageserverclient PR

See PR for detailed before/after exception counts in the speedometer run

Author: ToddGrun

src/LanguageServer/Protocol/Protocol/Internal/VSInternalContinueCharacterClass.cs

@@ -9,14 +9,18 @@ namespace Roslyn.LanguageServer.Protocol;
 /// <summary>
 /// Class representing a unicode character class for completion continuation.
 /// </summary>
+[Kind(Type, "_vs_type")]
 internal sealed class VSInternalContinueCharacterClass
 {
+    public const string Type = "unicodeClass";
+
     /// <summary>
     /// Gets the type value.
     /// </summary>
     [JsonPropertyName("_vs_type")]
     [JsonRequired]
-    public const string Type = "unicodeClass";
+    [JsonInclude]
+    internal string TypeDiscriminator = Type;
 
     /// <summary>
     /// Gets or sets the unicode class.

src/LanguageServer/Protocol/Protocol/Internal/VSInternalContinueCharacterRange.cs

@@ -9,14 +9,18 @@ namespace Roslyn.LanguageServer.Protocol;
 /// <summary>
 /// Class representing range of characters for completion continuation.
 /// </summary>
+[Kind(Type, "_vs_type")]
 internal sealed class VSInternalContinueCharacterRange
 {
+    public const string Type = "charRange";
+
     /// <summary>
     /// Gets the type value.
     /// </summary>
     [JsonPropertyName("_vs_type")]
     [JsonRequired]
-    public const string Type = "charRange";
+    [JsonInclude]
+    internal string TypeDiscriminator = Type;
 
     /// <summary>
     /// Gets or sets the first completion character of the range.

src/LanguageServer/Protocol/Protocol/Internal/VSInternalContinueCharacterSingle.cs

@@ -9,14 +9,18 @@ namespace Roslyn.LanguageServer.Protocol;
 /// <summary>
 /// Class representing single continue character for completion.
 /// </summary>
+[Kind(Type, "_vs_type")]
 internal sealed class VSInternalContinueCharacterSingle
 {
+    public const string Type = "singleChar";
+
     /// <summary>
     /// Gets the type value.
     /// </summary>
     [JsonPropertyName("_vs_type")]
     [JsonRequired]
-    public const string Type = "singleChar";
+    [JsonInclude]
+    internal string TypeDiscriminator = Type;
 
     /// <summary>
     /// Gets or sets the completion character.