Replace System.Text.Json with SimpleJSON

[danipen]

Summary

Removes System.Text.Json dependency and replaces it with SimpleJSON - a single-file, zero-dependency JSON parser.

Motivation

System.Text.Json pulls in multiple transitive dependencies for .NET Standard 2.0:

• Microsoft.Bcl.AsyncInterfaces
• System.Buffers
• System.IO.Pipelines
• System.Memory
• System.Runtime.CompilerServices.Unsafe
• System.Text.Encodings.Web
• System.Threading.Tasks.Extensions

This makes integration with Unity projects cumbersome.

Changes

• Added SimpleJSON.cs to TextMateSharp/Internal/Parser/Json/
• Rewrote JSONPListParser.cs to use SimpleJSON
• Rewrote GrammarDefinition.cs with manual parsing (removed [JsonPropertyName] attributes)
• Rewrote LanguageConfiguration.cs - replaced 6 custom JsonConverter<T> classes with direct parsing
• Updated RegistryOptions.cs to use new parsing methods
• Removed JsonSerializationContext.cs (source generator no longer needed)
• Removed System.Text.Json PackageReference from both .csproj files

Notes

• SimpleJSON handles trailing commas and // line comments (same as original)
• Added GetNullableString() helper to preserve null semantics (SimpleJSON returns empty string for missing keys)
• Memory usage slightly different: now loads full file vs streaming, acceptable for grammar files (<100KB)

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Disclaimer: I created this PR using Claude Opus 4.5