feat: add Loading component and loading states across components

[seanrcollings]

Summary

• Adds a new <Loading> ambient provider component and useLoading() hook, so a group of fields can be marked as loading from one place instead of threading isLoading through every child.
• Adds isLoading/loadingLabel support (skeleton loading states) to Input, TextArea, ComboInput, TextField, NumberField/NumberInput, SearchField/SearchInput, TextInput, TextAreaInput, CheckBox, ToggleSwitch, RadioGroup, FileInput, CustomSelect, MultiSelect, ComboBox, Table, and wires Button/IconButton/LoadingStatus up to the same ambient useLoading() context.
• Adds Storybook stories demonstrating the loading state for each of the above components, plus a new Loading stories file with simple examples and a more complex example form.
• Adds a "Loading States" tab to the playground consolidating all of the above (merging in the old standalone Loading tab).

Screenshots

Test plan

• npm run build --workspace @atomicjolt/atomic-elements passes
• Targeted typecheck of all new/edited Storybook stories passes
• Manual pass through the new "Loading States" playground tab and Storybook Feedback/Loading stories in a browser