Add "fallbackLanguage" to piral-translate

[manuelroemer]

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Description

Adds a new fallbackLanguage to piral-translate which allows configuring a default language to be used for translations prior to the default fallback function.

The goal of this PR is to allow translate API users to always show a translation with a conventional default language (e.g., english) that is always known to be available, even if the user's desired language is not yet implemented in a pilet.
Currently, with the default fallback, this is not (easily) possible because custom fallback functions do not have access to the Localizer's internal API or the formatMessage function.

Remarks

In principle, this PR is a suggestion - I am sure that there are other ways of implementing this. I am open to suggestions, this PR is just, in my eyes, the most straightforward way to do this.

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[github-actions]

File size impact

Impact on file sizes when merging feature/fallback-language into develop.

empty-piral (no impact)

Files	new size
Unmodified (2)	30.4 kB (0 B / +0%)	👻
Total (2)	30.4 kB (0 B / +0%)	👻

minimal-piral (no impact)

Files	new size
Unmodified (2)	280 kB (0 B / +0%)	👻
Total (2)	280 kB (0 B / +0%)	👻

sample-cross-fx (no impact)

Files	new size
Unmodified (3)	2.2 MB (0 B / +0%)	👻
Total (3)	2.2 MB (0 B / +0%)	👻

sample-piral (+0.03%)

Files	new size
…/samples/sample-piral/dist/release/index.cf29f2.js	528 kB	👶
…/samples/sample-piral/dist/release/main.93c1c5.css	171 kB	👶
…/samples/sample-piral/dist/release/main.960ccc.css	deleted (-171 kB)
…/samples/sample-piral/dist/release/index.97925c.js	deleted (-528 kB)
src/samples/sample-piral/dist/release/index.html	490 B (0 B / +0%)	👻
Total (5)	700 kB (+186 B / +0.03%)	↗️

sample-piral-core (no impact)

Files	new size
Unmodified (3)	357 kB (0 B / +0%)	👻
Total (3)	357 kB (0 B / +0%)	👻

_{Generated by @jsenv/file-size-impact during check-bundle-size#22401032677 on 9dbe975}

[FlorianRappl]

LGTM!