Set default board version

[Amerlander]

Legacy links cannot be imported: No board version is selected by default. The same issue occurs when the page is reloaded while the board selection is open.

1. Import https://makecode.calliope.cc/_MefTEdiLrACt into makecode.calliope.cc/beta.
2. Motor Blocks are not found, although they are still available in the library in the sidebar. However, once placed, they are also not found.

We have introduced board versions and moved some blocks from the core into the board packages. The default board version should be v1 for existing links.

I initially thought that the option "corepkg": "v1" in pxtarget.json was meant for that. However, that does not work. It affects the available blocks in the sidebar, but they are not found when used.

Another approach was to map the core package in the patches section. That leads to an infinite loop; therefore, I tried to rename core into core-mini and map this. However, a few libraries from pxt-common-packages require the core package to actually be named as core.

@abchatra, can you tell if there is something we can do to load the v1 package as the default/fallback?