Added line height css variable

[przemyslaw-zan]

Suggested merge commit message (convention)

Other (core): Added the --ck-content-line-height css variable. Closes #18634.

MAJOR BREAKING CHANGE: The editor now enforces a default line height of 1.5, affecting both editing and rendered content. This may impact existing styling and layout, so custom line-height settings should be reviewed.

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[pomek]

I would add a breaking change note to the changelog.

MAJOR BREAKING CHANGE: The editor now enforces a default line height of 1.5, affecting both editing and rendered content. This may impact existing styling and layout, so custom line-height settings should be reviewed.

[pomek]

Closing in favor of #18736.