Fluent: Catch strings with "[" at line start as a syntax error

[ayshushus]

Fix #3621

The idea is to effectively parse the translation as a single entry and then check whether there’s any non-whitespace left after that entry’s span.

The report is "unexpected text after the end of the message" and consideration for [*.}, see reference below:

http://localhost:8000/mua/common-voice/web/locales/common-voice/en/pages/contribute/common.ftl/?string=312854

[codecov-commenter]

Codecov Report

❌ Patch coverage is 91.07143% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.86%. Comparing base (c416aee) to head (2fd285c).

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[eemeli]

This is a syntax error which needs to be caught by moz-l10n, not in Pontoon.

[ayshushus]

Closing in reference to moz-l10n PR 222