Generate TypeScript definition files as part of the frontend-shared
package so that TS can typecheck code that uses the package in other
projects. Within the client repository we don't need to generate these files
because TypeScript will read the JSDoc comments. When the package is
consumed from other projects however, it seems that it does not read the
JSDoc comments. Note that when these `.d.ts` files exist, they are used in preference
to JSDoc comments by other code in the client repository. Therefore they
need to be kept up to date when the files are recompiled in watch mode.

In the process of adding this I discovered an issue that the method of
running CLI commands in gulp tasks in `scripts/gulp/frontend-shared.js` did not
fail if the exit status was non-zero. I created a `run` utility that
handles this correctly along with providing the desired defaults.