Summary

Convert the EuiIcon component to TypeScript. Notes:

• I switched the invocation of tsc and tslint, as TSLint recommends that compilation run cleanly before linting.
• I added a directory for custom library definitions, so that TS wouldn't complain about importing SVGs. These imports are still compiled into inline React components with Babel.
• I removed the icon title tests, since they have been deprecated and wrong for a long time.

Something else to note is that I'm seeing the following warning when building EUI now:

[BABEL] Note: The code generator has deoptimised the styling of /Users/rory/src/elastic/eui/src/components/icon/icon.tsx as it exceeds the max of 500KB.

True enough, if you look at the output in lib/, it's...ugly. I ran it through Prettier, and did the same for master, and the output varies in the expected ways but not by that much. Perhaps I just inched over the threshold? I did wonder if we should stop using the plugin and write a script to call svgo and generate actual component files instead? In any case, I started the docs site locally and the icons looked fine.

Checklist

• This was checked in mobile
• This was checked in IE11
• This was checked in dark mode
• Any props added have proper autodocs
• Documentation examples were added
• A changelog entry exists and is marked appropriately
• This was checked for breaking changes and labeled appropriately
• Jest tests were updated or added to match the most common scenarios
• This was checked against keyboard-only and screenreader scenarios
• This required updates to Framer X components