Convert this library to ES Modules

[willdurand]

We started to convert our libraries to ES Modules. This PR does that for
this package, which is written with TypeScript and obviously ESM isn't a
first class citizen there (yet?).

This is needed for converting addons-linter to ES Modules. Actually,
it is more of a nice to have but not required. That being said, converting
this package will allow us to keep up with the dependency updates.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.05 ⚠️

Comparison is base (5fbc283) 97.48% compared to head (46faad9) 97.43%.

❗ Current head 46faad9 differs from pull request most recent head e048960. Consider uploading reports for the commit e048960 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #244      +/-   ##
==========================================
- Coverage   97.48%   97.43%   -0.05%     
==========================================
  Files          11       11              
  Lines         437      429       -8     
  Branches       97       96       -1     
==========================================
- Hits          426      418       -8     
  Misses         11       11              

Impacted Files	Coverage Δ
src/constants.ts	100.00% <ø> (ø)
src/io/constants.ts	100.00% <ø> (ø)
src/io/utils.ts	100.00% <ø> (ø)
src/test-helpers.ts	97.05% <ø> (ø)
src/io/base.ts	97.72% <100.00%> (ø)
src/io/crx.ts	100.00% <100.00%> (ø)
src/io/directory.ts	100.00% <100.00%> (ø)
src/io/xpi.ts	96.03% <100.00%> (-0.08%)	⬇️

... and 1 file with indirect coverage changes

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[rpl]

just one questions related to the values sets in tsconfig.json, compared to what we have been setting in mozilla/sign-addon#1077.

[rpl]

lgtm, I was wondering: should we make sure we converted the other packages that depends from this one (e.g. addons-linter is one for sure, but I recall there was at least one or two more) before merging this and releasing a version of this package that is ESM-only?

If the way this is used in the other packages can be also just use the dynamic import, we may release the ESM-version of this package independently from the order we will do the same on the other ones, but I wanted to ask you explicitly in case we'll need to merge this and release a new version on npm after we have done the same kind of conversion on the dependents.

[willdurand]

As discussed on Slack, I am going to wait a bit and therefore mark this PR as a draft. I think ESM does not necessarily work well with pure TypeScript projects at the moment and this lib is used in one of them... I'll look into that next week or so.

[diox]

@willdurand with TypeScript 5 out, is is time to revisit this ?

[willdurand]

Yeah, I think the problem came from customs back then. If TS5 better supports ESM, we should be good.