[EuiIcon] Add plus and minus glyphs

[andreadelrio]

Summary

Adds plus and minus glyphs. This PR contains the option where the stroke width is 1px. I also explored the option of 1.5px stroke width as you'll see in the screenshots below (plus1_5 and minus1_5).

Fixes #1748

Checklist

• Check against all themes for compatibility in both light and dark modes
• Checked in mobile
• Checked in Chrome, Safari, Edge, and Firefox
  - [ ] Props have proper autodocs
• Added documentation
  - [ ] Checked Code Sandbox works for the any docs examples
  - [x] Added or updated jest tests
  - [ ] Checked for breaking changes and labeled appropriately
  - [ ] Checked for accessibility including keyboard-only and screenreader modes
• A changelog entry exists and is marked appropriately

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4111/

[thompsongl]

Just a note that the change to all icon files is a result of upgrading @svgr/core. This just happened to be the first time the icons were recompiled since the upgrade.

[cchaos]

Honestly, I know it goes against our typical glyph making rules, but the 1.5px stroke makes such a small icon so much easier to see. I even checked it on a non-retina screen, and yes, while it's not as "crisp" as the single pixel, it's still bolder and easier to see.

[thompsongl]

I'll let @cchaos have final say on the new icons, but the other 400 files changed LGTM 😄

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4111/

[cchaos]

Jenkins, test this

[cchaos]

Looks great to me! 👍

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4111/

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4111/