[rendering] update build script

[samelhusseini]

Updated build script to support 'core' and 'scratch' rendering.
Blocks are shared, 'core' will exclude the scratch rendering file and 'scratch' will exclude the core renderer (block_render_svg).

'core' generates blockly_compressed.js
'scratch' generates blockly_scratch_compressed.js

Thoughts on what we want to call this? Can probably re-visit that later.
I also added a scratch_playground which is a dupe of playground.html but references the blockly_scratch_compressed.js

Note: calling this script currently will fail as block_render_svg_scratch.js doesn't exist yet.
I've left accessible as is.

[rachel-fenichel]

Let's use 2018_rendering as the temporary name, instead of scratch. So blockly_2018_rendering_compressed.js, 2018_rendering as the flag, etc. We can revisit it to pick a proper name, but this can help us avoid some confusion in the interim.

The additional playground is fine. Eventually we'll make that an option on the main playground.

[samelhusseini]

Updated:
build.py 2018_rendering
Playground: 2018_rendering_playground.html
Render SVG file: block_render_svg_2018_rendering.js
Output: blockly_2018_rendering_(un)compressed.js