Update core bindings to Yrs v0.18

[LSViana]

With the release of Yrs v0.18, some core functions changed and should be updated in the library.

• [x] Replace *_unobserve_*() bindings with call to yunobserve()
  • Refers to: New Observer API #385@y-crdt
• [x] Remove yxmlelem()
  • [x] Fix unit tests
• [x] Remove yxmltext()
  • [x] Fix unit tests
• [x] Remove ybranch_write_transaction()
• [x] Remove ybranch_read_transaction()
• [x] Create bindings for YBranchId:
  • [x] ybranch_id()
  • [x] ybranch_get()
  • [x] ybranch_alive()
  • Refers to: Logical pointers #393@y-crdt

All items above include the bindings and unit tests.

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Breaking changes

• Remove Doc.XmlElement()
  • Fix: use Doc.XmlFragment() for root-level types and then create an XmlElement inside of the XmlFragment.
• Remove Doc.XmlText()
  • Fix: use Doc.XmlFragment() for root-level types and then create an XmlText inside of the XmlFragment.

[LSViana]

@SebastianStehle FYI, I'll work on this.

[SebastianStehle]

Awesome. I am looking forward to the new branch pointers :)