Update TypeScript and enable strict mode

[adroitwhiz]

Resolves #92
Resolves #97

This PR updates TypeScript to the latest release (4.9), updates Prettier to the next major version, and enables strict mode.

Some externally-visible changes have been made to the codebase:

• Sound.sampleCount and Sound.sampleRate are now nullable
• Numeric fields are now always serialized to .sb3 as strings
• When deserializing, null block inputs are skipped completely for now
  • I'll revisit this later. If there's, for instance, an empty C-block inside a .sb3 file, its SUBSTACK will be either null or completely absent in the .sb3's block inputs, depending on whether a stack was ever dragged into the C-block. We previously used to treat the two cases differently, treating the input as {type: "string", value: null} in the former case (flat-out wrong), and leaving the input out in the latter case. Now, we always leave the input out. It'll be easier to fix this properly once we have a better way to enumerate a block's intended inputs-- see Proposal: New API + set of types for working with blocks in a type-safe way #93.
• Null procedure_call arguments are now {type: 'boolean', value: false}, which matches what they are when exported from Scratch.

I haven't touched the Leopard serialization code yet, since #90 touches it a lot. I'll go over it and un-draft this PR once it's merged.

[adroitwhiz]

PS: While any (existing) code style issues can be noted, I don't think they should be blockers here. I see this PR as a prerequisite to other changes (including fixing those issues), and fixing #100 will mean re-coding a substantial portion of this project regardless, which will likely bulldoze over any code style issues.

[towerofnix]

Thanks, this looks awesome! I appreciate how much more guarded strict mode is over null/undefined, and I'm assuming it also played a role in the rest of the improvements here, which look great. 🚀

I have just a few remarks and one or two minor requested changes — this PR is pretty much good-to-go as is. Thanks for all your work on it!

[towerofnix]

Looks good! Thank you!

[towerofnix]

Since we've gone through a thorough review of this earlier, I'm calling this good to merge.

There are loads of force pushes above on my part - sorry about that! These were to fix unaddressed lint and snapshot issues in-place as they cropped up, rather than later commits. The history should be good here now, it just took a couple tries to get right. ^^;