Add an option to output detected issues as JSON about checksync

[somewhatabstract]

I think the file should also include the arguments used to execute checksync so that it contains all the data needed to recreate the run. That way, if we so chose, we could update checksync to read this format too and know exactly how to interpret all the paths and such.

[somewhatabstract]

Also, including the version of checksync would be useful - just in case we decided to change the format.

[kevinbarabash]

I think the following structure for the JSON output could work:

[{
   type: "error",
   file: "services/static/javascript/app-shell-package/app-entry.js",
   line: 14,
   message: "Sync-start for 'app-shell-entry' points to 'services/static/webpack/plugins/make-i18n-html-pages.jsx', which does not exist or is a directory",
}, {
   type: "mismatch",
   file: "services/static/javascript/app-shell-package/app-entry.js",
   line 14,
   message: "Looks like you changed the target content for sync-tag 'app-shell-entry' in 'services/static/webpack/plugins/make-i18n-html-pages.js:80'. Make sure you've made the parallel changes in the source file, if necessary (0 != 2118575972)",
   fix: "// sync-start:app-shell-entry 2118575972 services/static/webpack/plugins/make-i18n-html-pages.js",
}]