[ui-components] create top-level module declaration

[Joe Lowinske]

In order to be imported by external Typescript projects, ui-components needs a module declaration file to be included in the output bundle. Relevant links and discussions:

• publishing declarations in your NPM package
• generate per-component declaration files by setting declaration: true in TS compiler options)
• discussion re: proposal for supporting declarations bundling
• a couple options for external packages that could potentially help (1, 2)