Add no <style> tags test

[larouxn]

Add a test to ensure we don't add SVGs with <style> tags. Note, should allow inline styles.

Note, fails for some current SVGs as they have <style> tags. Maybe we should create a "legacy exceptions list". 🤔

Enforces the following line in the CONTRIBUTING.md.

All styles should be inline. There should be no <style> tag unless it is absolutely necessary.

Resolves #179
Part of resolving #184

[larouxn]

Closing this, at least for now, as I don't currently have time.