adding a few tutorial options and fixing toolbox filters

[riknoll]

part of the fix for microsoft/pxt-arcade#7012
part of the fix for microsoft/pxt-arcade#7013
part of the fix for microsoft/pxt-arcade#7010

adds a few tutorial features/fixes for bug arena:

• new hiddenNamespaces snippet in tutorial markdown that lets you edit the toolbox filters in the tutorial project. this is a useful alternate method of filtering the toolbox that lets you remove categories without having to enumerate every single block from the categories you want to keep inside a ghostBlocks snippet
• new ### @hideReplaceMyCode true config that will hide the "replace my code" button in skillmap tutorials
• fixes a bug where the toolbox filters in a project's pxt.json could not filter the variables or functions categories from the toolbox
• tweaks the code carryover behavior so that carryover is allowed without a templateCode snippet iff the "replace my code" button is hidden

this PR adds all of the pxt-core fixes for the above issues, but doesn't actually fix them since the tutorial authoring also needs to change. i'll have a separate pr in pxt-arcade with the authoring changes once this is merged/hotfixed