Add modbus hvac_* write registers

[janiversen]

Breaking change

Proposed change

HVAC_MODE and HVAC_ONOFF are both single registers in modbus term, but some devices needs the use of write_registers instead of write_register.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
  fixes Problem with modbus climate for mc6 thermostat #86469
• This PR is related to issue:
• Link to documentation pull request:
  Add modbus write_registers keys home-assistant.io#26595

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
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• The code has been formatted using Black (black --fast homeassistant tests)
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io

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• The manifest file has all fields filled out correctly.
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• New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
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• Untested files have been added to .coveragerc.

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[home-assistant]

Hey there @adamchengtkc, @vzahradnik, mind taking a look at this pull request as it has been labeled with an integration (modbus) you are listed as a code owner for? Thanks!

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[MartinHjelmare]

Thanks!