[Repo Assist] fix(js): handle .NET format specifiers in F# interpolated strings

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Closes #4046 (items 3 and 4 — $"{x:N0}" and $"{n:#,#} items" producing literal %P(N0) / %P(#,#) output).

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Root cause

When an F# interpolated string uses a .NET format specifier such as $"{x:N0}", the F# compiler encodes the hole in the PrintFormat format string as %P(N0) — with the specifier inside the parentheses.

The regex in makeStringTemplateFromWith (Replacements.Util.fs) was %P\(\), which only matched empty parentheses. The pattern %P(N0) was never recognised as a hole, so:

1. makeStringTemplateFrom incorrectly built a StringTemplate that contained the literal text %P(N0) with no interpolated value — a silent, wrong result.
2. makeStringTemplateFromAllowingFormat (the second attempt) never got a chance to apply formatting because the first attempt returned Some(brokenTemplate) instead of None.

The end result was the literal string %P(N0) appearing in the JavaScript output.

Fix

• src/Fable.Transforms/Replacements.Util.fs

  • Update the regex from %P\(\) to %P\(([^)]*)\) to also capture the optional .NET specifier inside the parens (captured in Group 2).
  • Add a handleDotNetSpec: string -> Expr -> Expr option callback parameter to the private makeStringTemplateFromWith.
  • makeStringTemplateFrom passes (fun _ _ -> None) for handleDotNetSpec, so it correctly returns None when a .NET spec is present, falling through to makeStringTemplateFromAllowingFormat.
  • makeStringTemplateFromAllowingFormat passes a callback that calls String.format("{0:<spec>}", value) — the same library function that already handles (1000).ToString("N0").

• tests/Js/Main/StringTests.fs

  • Two new test cases covering N0, N2, F2 standard numeric specifiers and the #,# custom pattern, including mixed strings like $"Count: {n:N0} items".

Trade-offs

• Python, Dart, and BEAM targets still fall through to their runtime String.interpolate for these cases (which also doesn't handle .NET specs) — that is a pre-existing limitation and left for a separate fix. This PR only targets JavaScript/TypeScript.
• No changes to src/Fable.AST/ or src/Fable.Core/.
• No changelog update — the changelog is auto-generated from commit history per project convention.

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