feat: add JSON language server support via vscode-json-languageserver (#1404)

* feat: add JSON language server support via vscode-json-languageserver (fixes #1391)

Add support for JSON files using the `vscode-json-languageserver` npm package
from Microsoft. This provides document symbol navigation and hover information
for JSON files.

The implementation follows the existing YAML language server pattern:
- Auto-installs `vscode-json-languageserver` via npm on first use
- Requires Node.js and npm
- Marked as experimental (must be explicitly specified)

Co-Authored-By: Octopus <liyuan851277048@icloud.com>
Co-authored-by: octo-patch <octo-patch@github.com>
Co-authored-by: Michael Panchenko <35432522+MischaPanch@users.noreply.github.com>

README.md

@@ -110,7 +110,7 @@ Serena incorporates a powerful abstraction layer for the integration of language
 The underlying language servers are typically open-source projects or at least freely available for use.
 
 When using Serena's language server backend, we provide **support for over 40 programming languages**, including
-AL, Ansible, Bash, C#, C/C++, Clojure, Crystal, Dart, Elixir, Elm, Erlang, Fortran, F#, GLSL, Go, Groovy, Haskell, Haxe, HLSL, Java, JavaScript, Julia, Kotlin, Lean 4, Lua, Luau, Markdown, MATLAB, mSL, Nix, OCaml, Perl, PHP, PowerShell, Python, R, Ruby, Rust, Scala, Solidity, Swift, TOML, TypeScript, WGSL, YAML, and Zig.
+AL, Ansible, Bash, C#, C/C++, Clojure, Crystal, Dart, Elixir, Elm, Erlang, Fortran, F#, GLSL, Go, Groovy, Haskell, Haxe, HLSL, Java, JavaScript, JSON, Julia, Kotlin, Lean 4, Lua, Luau, Markdown, MATLAB, mSL, Nix, OCaml, Perl, PHP, PowerShell, Python, R, Ruby, Rust, Scala, Solidity, Swift, TOML, TypeScript, WGSL, YAML, and Zig.
 
 ### The Serena JetBrains Plugin