TS 4.9 compat

[markerikson]

• Added TS 4.7, 4.8, and 4.9 RC to the test matrix
• Bumped the in-repo TS version to 4.8
• Reconfigured all the types so that TS 4.6 and earlier get the existing MergeParameters implementation (which actually worked up through 4.8), whereas 4.7+ get the new implementation from Consistently add undefined/missing to optional tuple element types microsoft/TypeScript#50831 (comment)
  • This required stupid hacky build-time package shenanigans that we previously did over in the Redux Toolkit repo to handle TS <4.1. We removed those changes in drop pre-TS-4.1 types and workarounds redux-toolkit#2249 in preparation for RTK 1.9 - see the removed code in that PR

Pasting the main comment from the big commit:

Per microsoft/TypeScript#50831 , the existing implementation of MergeParameters broke with TS 49 due to a change in how "optional" and "undefined" arguments get interpreted

Anders Hjelsberg himself supplied a new implementation that's actually much simpler, but only works with TS 4.7 and greater.

Normally this would require shipping two entirely different sets of
types, as we already do for TS <4.1. However, there's a trick we used
with RTK where we:

• Create a folder and put two different files with different impls of the same type inside
• Add an index file and re-export one of those
• Add a file named package.dist.json containing typesVersions that points to both of those .d.ts files
• Copy that package.dist.json over to dist/some/package.json during the actual build/publish step

That way during dev TS just imports the type as normal, but the built
version sees that some/package.json, sees typesVersions, finds the
right .d.ts file, and imports the right implementation for itself.

[codecov]

Codecov Report

Merging #587 (474b12c) into master (ae67c1f) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 474b12c differs from pull request most recent head 11ed107. Consider uploading reports for the commit 11ed107 to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #587   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            2         2           
  Lines          105       105           
  Branches        24        24           
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  Hits           105       105           

Impacted Files	Coverage Δ
src/index.ts	100.00% <100.00%> (ø)

[markerikson]

Confirmed this works by:

• Creating a new Vite React+TS app project
• Copying the entire typetest file into that
• doing a yalc publish from this branch and installing into that repo
• taking turns installing TS 4.2, 4.6, 4.8, and 4.9-rc and running yarn tsc to confirm that the typetest file compiles cleanly
• uncommenting the dead chunk around like 615 where TS 4.8 and earlier had a // @ts-expect-error, and confirming that 4.9 panics because there isn't an error (also confirming that they all really were compiling the file)