Changes since v6.1.2

**monorepo**:
* Use blazediff to speed up visual diffs (thanks @teimurjan!) (via
#4125)
* Update version in specs (thanks @domoritz!) (via #4045)

**vega**:
* Export schema from Vega package (thanks @domoritz!) (via #4046)

**vega-loader**:
* Correct mode handling in loader (thanks @domoritz!) (via #4048)

**vega-util**:
* Fix handling of null values in stringValue (thanks @domoritz!) (via
#4127)
* Replace all problematic Unicode characters (thanks @basbroek!) (via
#4130)

**docs**:
* Update links to Prefuse and Protovis (thanks @star1327p!) (via #4095)
* Replace version in specs (thanks @domoritz!) (via #4085)
* Update file name in weather example (thanks @domoritz!) (via #4083)
* Update versions in docs (thanks @domoritz!) (via #4050)
* Update of the labelled donut example (thanks @PBI-David!) (via #4064)
* Addition of the labelled donut example (thanks @PBI-David!) (via
#4062)

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Signed-off-by: Lukas Hermann <1734032+lsh@users.noreply.github.com>