Add and use length millimeters constant

[springstan]

Breaking change

• Add length millimeters constant to HA constants
• Use the newly added constant in the entire code base

Proposed change

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• Bugfix (non-breaking change which fixes an issue)
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[vangorra]

LGTM

[MatthewFlamm]

Should this also be added to homeassistant.util.distance?

[springstan]

Should this also be added to homeassistant.util.distance?

Good question. I will open another PR to not only add millimeters, but also other length units that are not considered in the convert() function yet.

[dgomes]

All good!