Fix regex validator crash on invalid patterns in jquery.validate.unobtrusive.js

## Issue

The `regex` custom validator in `jquery.validate.unobtrusive.js` (shipped with ASP.NET Core Identity UI) throws a `SyntaxError` when `new RegExp(params)` encounters an invalid regex pattern from `data-val-regex-pattern`. This crashes all client-side form validation on the page.

## Steps to Reproduce

A `[RegularExpression]` attribute with an invalid pattern (e.g., unbalanced brackets) generates a `data-val-regex-pattern` attribute. When jQuery Validate evaluates `new RegExp(params)`, it throws.

## Proposed Fix

Wrap the `new RegExp()` call in a try-catch so invalid patterns skip validation gracefully instead of crashing. Server-side validation at form submission remains the authoritative fallback.

Found via static analysis of the ASP.NET Core codebase.