fix(js/ts): reject JS-permissive date strings that .NET TryParse rejects

[fable-repo-assist]

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Summary

Fixes DateTime.TryParse accepting strings in JavaScript that .NET correctly rejects.

Closes #3858

Root Cause

V8's Date constructor is overly permissive — it accepts strings like "ABC 6" by treating "ABC" as a timezone abbreviation and "6" as a year/month. .NET's DateTime.TryParse rejects such strings because they don't match any recognised date format pattern.

In parseRaw(), the code calls new Date(input) first and only falls back to a strict regex when isNaN(date.getTime()). Since V8 successfully parses "ABC 6", the guard never fires.

Fix

Add a pre-validation check at the top of parseRaw() that requires the string to start with either a digit or a recognised month-name prefix (Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec — case-insensitive):

if (!/^\s*(?:\d|Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec)/i.test(input)) {
    fail();
}

This lets valid formats through:

• Numeric dates: "9/10/2014", "2024-01-15", "2024-01-15T12:00:00Z"
• Month-name dates: "January 15, 2024", "Dec 25 2024"
• Time-only: "12:00:00"

And rejects V8-specific quirks:

• "ABC 6" → fails (starts with A, not a digit or month name) ✓
• "XYZ 2024" → fails ✓
• "foo" → still fails (already tested) ✓

Changes

• src/fable-library-ts/Date.ts: add pre-validation in parseRaw()
• tests/Js/Main/DateTimeTests.fs: add regression test for "ABC 6" and "XYZ 2024"

Trade-offs

The regex is simple and only checks the start of the string. It does not attempt to fully validate the date format (that remains the job of new Date() and the fallback regex). This minimises the risk of breaking existing valid inputs while fixing the specific V8 over-acceptance bug.

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