[FEATURE]: Add built-in StyLua formatter

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Describe the enhancement you want to request

OpenCode has built-in Lua language-server support, but no built-in Lua formatter. Users who already have StyLua installed must repeat it as a custom formatter command in every configuration.

Add a built-in stylua formatter that detects the executable on PATH and formats both .lua and .luau files in place. Include regression coverage proving the built-in extensions are retained when the command is configured and that both file types are formatted.