feat(strings): implement CharacterMappingLoader for JSON-based character mappings

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Co-Authored-By: Claude <noreply@anthropic.com>

src/Umbraco.Core/Strings/CharacterMappingFile.cs

@@ -0,0 +1,27 @@
+namespace Umbraco.Cms.Core.Strings;
+
+/// <summary>
+/// Represents a character mapping JSON file.
+/// </summary>
+internal sealed class CharacterMappingFile
+{
+    /// <summary>
+    /// Name of the mapping set.
+    /// </summary>
+    public required string Name { get; init; }
+
+    /// <summary>
+    /// Optional description.
+    /// </summary>
+    public string? Description { get; init; }
+
+    /// <summary>
+    /// Priority for override ordering. Higher values override lower.
+    /// </summary>
+    public int Priority { get; init; }
+
+    /// <summary>
+    /// Character to string mappings.
+    /// </summary>
+    public required Dictionary<string, string> Mappings { get; init; }
+}