Add contrast colors to `theme-colors.yml`

[MartijnCuppens]

In our docs we check for the name of the colors to apply a light or dark color:

https://github.com/twbs/bootstrap/blob/master/site/content/docs/4.3/utilities/colors.md#L33

We could add this to the theme-colors.yml file like this:

- name: primary
  hex: "#007bff"
  contrast_color: white
- name: secondary
  hex: "#868e96"
  contrast_color: white
- name: success
  hex: "#28a745"
  contrast_color: white
- name: danger
  hex: "#dc3545"
  contrast_color: white
- name: warning
  hex: "#ffc107"
  contrast_color: dark
- name: info
  hex: "#17a2b8"
  contrast_color: white
- name: light
  hex: "#f8f9fa"
  contrast_color: dark
- name: dark
  hex: "#343a40"
  contrast_color: white

This will be easier to maintain since we don't have to check for light or warning everywhere.

[XhmikosR]

That or have text-white as a default and only specify the exceptions.

[mdo]

Would #29348 resolve this by using the new swatch docs colors and color contrast checking built into them?

[ffoodd]

Came across this issue while working on #30468: is it still relevant? I guess so, since I had to abuse of {{ else if …}} in my own PR.

May we improve or merge #30044 while at it? @XhmikosR you're the one who tried to improve this PR, any thought?

[XhmikosR]

I'll have a look at #30044 later.

[mdo]

Can we close this for #30044?