feat: add isSquare prop to enable square icons

[ali-ajam]

#401 This pull request introduces a new boolean prop, isSquare, to the SocialIcon component. With this prop, users can easily choose to render square icons instead of the default circular ones.

[couetilc]

@ali-ajam thank you for your effort. I'd like you to make some changes.

1. Rename the prop to square e.g. <SocialIcon square /> or <SocialIcon square={true} />.

2. Remove the changes made to test/visual/spec.jsx. They are unnecessarily duplicative. That page is testing the icon definitions, not the border-radius css rule.

3. In src/component.jsx, instead of using spread syntax to merge the key, simply set it conditionally in the body of the function e.g. if (square) social_svg.borderRadius = 0

Good job on the unit tests.

[ali-ajam]

@ali-ajam thank you for your effort. I'd like you to make some changes.

1. Rename the prop to square e.g. <SocialIcon square /> or <SocialIcon square={true} />.
2. Remove the changes made to test/visual/spec.jsx. They are unnecessarily duplicative. That page is testing the icon definitions, not the border-radius css rule.
3. In src/component.jsx, instead of using spread syntax to merge the key, simply set it conditionally in the body of the function e.g. if (square) social_svg.borderRadius = 0

Good job on the unit tests.

Thanks @couetilc, done.