refactor: use WeakMap to store/retrieve formats

[kricsleo]

resolves #54

[codecov]

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[pi0]

Thanks for PR and sharing benchmark.

Keeping references in local scope of package makes it not working if there are two versions of confbox used (can be same version but in different places even like one bundled) also init of map in outside makes treeshaking harder.

While both solvable, I think considering there is no significant difference in perf it is better to not do this.