[EuiIcon] Improve data URI type performance

[thompsongl]

Summary

Closes #5741

Running an "ends with" regex on a large string (such as a image data URI) takes a long time

Before

So using alternative string search methods help immensely

After

We could use indexOf instead of endsWith, but there's extra logic involved to prevent -1 matches. We're using the fastest method to bypass additional parsing of data URIs so I'm less concerned with perf after that point. endsWith provides a good bump compared to the regex (.1ms vs 2ms for URL strings).

Also added a data URI example to the docs.

Checklist

• Checked in Chrome, Safari, Edge, and Firefox
• Added documentation
• Checked Code Sandbox works for any docs examples
• Checked for breaking changes and labeled appropriately
• A changelog entry exists and is marked appropriately

[cee-chen]

[optional / can be done in a follow-up PR] it would be awesome to write a quick set of unit tests (should be 4-5 cases?) for the new util, but only if we have time / are waiting around for the Kibana PR to merge! if not we can add it in a follow-up PR

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_5751/