Update clangd config file schema with new config options in clangd 21

[HighCommander4]

Area with issue?

JSON Schema

✔️ Expected Behavior

When editing a .clangd configuration file, config keys added in clangd 21 are recognized.

❌ Actual Behavior

An error is reported for newly added config keys, e.g. "Property CodePatterns is not allowed".

JSON, YAML, or TOML file that does not work.

Completion:
  CodePatterns: None

IDE or code editor.

Visual Studio Code

[HighCommander4]

@hyperupcall it's that time of the year again :) Are you interested in updating the schema with new config keys in clangd 21? They are:

• BuiltinHeaders, under CompileFlags
• HeaderInsertion, under Completion
• CodePatterns, also under Completion