fix(isFloat): fall back to '.' separator for an unknown locale

[yfwmaniish]

Closes #2862.

Bug

isFloat interpolates the locale separator straight into a new RegExp:

const float = new RegExp(`^(?:[-+])?(?:[0-9]+)?(?:\${options.locale ? decimal[options.locale] : '.'}[0-9]*)?...`);

When options.locale is not a key of decimal, decimal[options.locale] is undefined, which stringifies into the pattern as the literal text undefined. The preceding \ yields \undefined, and since \u isn't followed by four hex digits it degrades to a literal u, so the "decimal separator" becomes the 8-character string undefined. There's no error and no fallback:

isFloat('3undefined5', { locale: 'no-such' }); // true  ❌
isFloat('3.5',         { locale: 'no-such' }); // false ❌

Fix

Fall back to . when the locale is unknown:

const decimalSeparator = decimal[options.locale] || '.';

Known locales are unaffected (their separator is always . or ,, both truthy), and the no-locale path already resolved to ..

Tests

Added an unknown-locale block asserting ordinary decimals validate and the undefined-separator strings no longer pass. Verified known-locale behavior (en-US ., de-DE ,) is unchanged.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (a79ff98) to head (ab967e2).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2878   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          114       114           
  Lines         2599      2600    +1     
  Branches       658       658           
=========================================
+ Hits          2599      2600    +1     

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