Added consts for better team usability

[s-krawczyk]

No description provided.

[jason-roberts]

Hi @s-krawczyk - thanks for the PR, I like the idea of "team constants" so there can be consistency if required. I'm not sure whether these should be baked into the library or not though, I think it would be ok to do this though. It should be noted that there's noting preventing a developer from just using any value in the attribute usage rather than a constant.

Just a couple of minor changes would be cool for the PR:

1 Can you delete the Pain.cs and just use the numeric constants in LegoPain.cs itself

2 In EffortToFix can we replace the abbreviations with actual words, e.g. Medium rather than just M

3 Is there a more specific name we can think of for EffortToFix, like TShirtEffort RelativeEffort or something to enable other effort classification constant schemes to be introduced in the future?

4 Not sure why but all the lines in the files are showing up as different in the PR diff, makes it hard to review the PR in GitHub.

Thanks again for the contribution :)