Add normally closed jumpers and 3P3P transformer

[eto-]

SUMMARY

I added:

• normally closed jumper (for default setting) to be cut in PCB
• a 3P3P transformer

DATASHEETS / REFERENCES

OPEN QUESTIONS / UNRESOLVED ISSUES

CHECKLIST

• I have read and followed the library conventions¹.
• For packages, I followed IPC7351C (see details in library conventions).
• For modified elements, I bumped their version number².
• I already used the new library elements in a design and verified their correctness (optional).
• I'm the copyright owner of the added content (i.e. the changes are made by myself, not copied/imported from somewhere else).
• I agree to publish all my changes under the CC0 Public Domain License³, allowing everyone to use and modify the content without any restrictions.

¹ Library Conventions: https://docs.librepcb.org/#libraryconventions
² Minor version bump if only metadata was modified (e.g. "0.1" -> "0.1.1"), major version bump if functional changes were made (e.g. "0.1" -> "0.2")
³ CC0 Public Domain License: https://en.wikipedia.org/wiki/CC0

[ubruhin]

Sorry for the late review! I checked them now and it looks quite good 🎉 I just added a few minor adjustments (see commit), hope you are fine with this.

Thanks for the contribution!