refactor: use block styles instead of colors

[gonfunko]

This is one of three PRs across scratch-blocks, scratch-gui and scratch-vm that refactor each package to define block colors as Blockly BlockStyles, instead of using the colour attribute in Block JSON. This has several advantages:

• Using the colour attribute causes the secondary/tertiary colors to be calculated, and Blockly does so in a way that slightly skews from the colors Scratch specifies. Scratch had previously modified core Blockly to read the secondary/tertiary colors out of the block JSON, but this isn't directly achievable using supported APIs.
• Using block styles makes it easy to switch themes, and Scratch's themes (currently just the default and a high contrast variant) are now transformed into BlockStyles/Blockly themes.

In addition, these changes fix the coloration of dropdown fields in their various forms to align with Scratch's behavior. The ability for Scratch extensions to specify a custom block color is also preserved, even though that functionality is currently unused.