Expose icons as named exports for consumers of package

[jeronimomora]

I quite like the Icons in this package. I wanted to create my own custom AlertTemplate, but still make use of the icons without doing a copy paste of code from this repository.

This pull request does a small refactor which bunches up all the icons as named exports from an index file in the 'src/icons' directory and moves the AlertTemplate to its own file. The 'src/index' file is then changed to be the entry point for the build, and is where AlertTemplate is exported as default, and the icons are exported as named exports.