Migrate flat, flat-square, plastic and social to use XmlElement (ba…

…dges#6883)

* start changelog entry for v4

* migrate Flat/FlatSquare/Plastic to use XmlElement

* move brightnessThreshold into colorsForBackground

* move old renderLogo function inline into social()

this is the only place it is now used

* use XmlElement in social()

* don't quote numbers if we don't need to

* remove intermediate calls to .render()

leave everything as XmlElement objects right till the end
then make one final call to .render()
which cascades aaall the way through the tree

* factor out code for assembling logo element

* use scale consts in social

* remove NullElement

now we've removed all the intermediate calls to render()
we can just use an empty string

* write leftlink so it doesn't look like a bool

Co-authored-by: Caleb Cartwright <calebcartwright@users.noreply.github.com>

badge-maker/CHANGELOG.md

@@ -1,5 +1,9 @@
 # Changelog
 
+## 4.0.0 [WIP]
+
+- Drop compatibility with Node 10
+
 ## 3.3.1
 
 - Improve font measuring in for-the-badge and social styles