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>

pyproject.toml

@@ -336,6 +336,7 @@ markers = [
   "haskell: Haskell language server tests",
   "haxe: Haxe language server tests",
   "yaml: language server running for YAML",
+  "json: language server running for JSON (uses vscode-json-languageserver)",
   "powershell: language server running for PowerShell",
   "pascal: language server running for Pascal (Free Pascal/Lazarus)",
   "cpp: language server running for C/C++",