feat: add support for ts 4.2

[RebeccaStevens]

PR Checklist

• Addresses an existing open issue: fixes 🐛 Bug: TypeScript 4.2 not supported #72
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Adds support for old versions of TypeScript, down to 4.2

[codecov]

Codecov Report

Merging #65 (a23c63a) into main (129563e) will decrease coverage by 0.11%.
The diff coverage is 13.04%.

@@            Coverage Diff             @@
##             main      #65      +/-   ##
==========================================
- Coverage   21.63%   21.52%   -0.11%     
==========================================
  Files          17       18       +1     
  Lines         416      432      +16     
  Branches      166      172       +6     
==========================================
+ Hits           90       93       +3     
- Misses        303      316      +13     
  Partials       23       23              

Flag	Coverage Δ
4.3.5	21.52% <13.04%> (?)
4.4.4	21.52% <13.04%> (?)
latest	21.52% <13.04%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/nodes/typeGuards/union.ts	1.94% <0.00%> (-0.24%)	⬇️
src/tokens.ts	0.00% <0.00%> (ø)
src/utils.ts	75.00% <75.00%> (ø)

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[JoshuaKGoldberg]

I defer to your preference - this is all wonderful either way! Thanks! 🔥

[RebeccaStevens]

Do we want to temporarily remove testing on typescript@next as the test are failing?

[JoshuaKGoldberg]

I think what you have now with the isTsVersionAtLeast function referring to ts.versionMajorMinor is probably my favorite approach: no semver dependency; directly asks the typescript package; is clear and understandable.

That being said, at this point I'm happy with whatever approach you want 😄. This package is still pretty new and 0.0.x level. We can always change if we find a need later on. I'd rather experiment in "production" and get it out sooner.

And yeah, a followup issue for typescript@next sounds good to me.

Great stuff either way - let's ship! 🚀 (let me know if you don't have merge permissions, you should)

[RebeccaStevens]

@JoshuaKGoldberg I don't have permission to override those required checks that no longer exist.