Allow passthrough of xgettext's plural templates

[akx]

As #1154 reported, Babel would choke trying to parse a plural expression that is but a template.

After this, such templates can be parsed and manipulated, but may have strange interactions with other features.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.68%. Comparing base (b50a1d2) to head (6cf8070).
⚠️ Report is 43 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@## master #1185 +/- ##
=======================================
Coverage 91.68% 91.68% =======================================
Files 27 27 Lines 4689 4690 +1 =======================================
+ Hits 4299 4300 +1 
Misses 390 390 

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[tomasr8]

After this, such templates can be parsed and manipulated, but may have strange interactions with other features.

I wonder if it'd be better to either reject these (with a nicer error message) or try to detect the nplurals=INTEGER; plural=EXPRESSION; snippet and default to nplurals=2 and plural=(n != 1) as we do now?

This way the catalog wouldn't be in this weird partially valid state. For type-checked code you also wouldn't need to type guard num_plurals in case it's a string