Make Alert and Compiler-Picker widgets (#4366)

static/formatter-registry.ts

static/main.js

static/panes/compiler.ts

static/panes/conformance-view.ts

static/panes/editor.ts

static/panes/executor.js

static/panes/tree.ts

static/widgets/alert.interfaces.ts

static/widgets/alert.ts

static/widgets/compiler-picker.ts

static/widgets/load-save.ts

static/widgets/pane-renaming.ts