Raise error in parse_arg_id if a dot if found in the variable name

[MichaelHoste]

We noticed that an ICU message looking like this is wrongly considered as a valid message:

{ count, plural, one {{results.length} result} other {{results.length} results} }

Note the use of the variable {results.length} instead of {count}, it could happen if the developer make a mistake while writing the ICU.

We checked and the same ICU triggers an error when parsed by the JS library @formatjs/icu-messageformat-parser

We improved a bit the parse_arg_id method to also raise an exception when dot (.) is found in a variable.
I hope it made sense.

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Thank you for this gem that is really useful to us at Translation.io 🙂