Add debugger support (DAP)

[TristanSpeakEasy]

This adds debugger support to goja using the Debug Adapter Protocol (DAP), so you can step-debug JavaScript/TypeScript running in goja from VS Code.

The core API lives in debugger.go and vm_debug.go — breakpoints (line, conditional, hit count, log points), stepping, pause, variable inspection, eval while paused, exception breakpoints, and source map support. The DAP server is in a separate debugger/ submodule to keep the go-dap dependency out of the main module.

When no debugger is attached there's zero overhead — everything is gated behind vm.dbg != nil.

The changes to existing files are kept minimal: compiler.go/compiler_expr.go/compiler_stmt.go emit debug variable maps when debugMode is set, runtime.go adds the attach/detach API and CompileForDebug, and vm.go dispatches to the debug-aware execution loop and hooks into save/restore context for frame tracking.

A VS Code extension is included in debugger/vscode-goja-debugger/ with a pre-built .vsix.

Tests: 33 tests for the core debug API in debugger_test.go, 17 DAP integration tests in debugger/server_test.go.

Note: The debugger/go.mod currently has a replace github.com/dop251/goja => ../ directive so it compiles against the local parent module during development. Once this PR is merged, that replace should be removed and the github.com/dop251/goja dependency updated to point at the merged commit (or a tagged release).