Add extra keymap for EurKEY layout

[j-hap]

Description

QMK contains a keymap_language.h for many OS keyboard layouts. There is non for EurKEY (https://eurkey.steffen.bruentjen.eu/layout.html). Now there is. I used the US Intl as reference, so there are lots of defined, that only change the KC prefix to EU. There are multiple versions of the EurKEY layout. I used 1.2 since 1.3 is marked beta and my current linux seems to contain 1.2.
No custom sendstring.h is needed, because all ASCII is the same as US ANSI.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[github-actions]

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[KarlK90]

Hi @j-hap there are merge conflicts, would mind rebasing your PR?