[JS/TS] Fix N format specifier producing trailing dot when precision is 0

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Summary

Fixes #2582 — ToString("N0") and String.Format("{0:N0}", ...) produce "1,000." (with a trailing dot) instead of "1,000".

Root Cause

In src/fable-library-ts/String.ts, the N format case always appended a decimal separator and decimal digits:

case "n": case "N":
  precision = precision != null ? precision : 2;
  rep = toFixed(rep, precision);
  parts = splitIntAndDecimalPart(rep);
  rep = thousandSeparate(parts.integral) + "." + padRight(parts.decimal, precision, "0");
  //                                        ^^^--- always appended even when precision=0
  break;

When precision = 0, toFixed(1000, 0) gives "1000", splitIntAndDecimalPart gives {integral: "1000", decimal: ""}, and the code produced "1,000." (thousands separator applied correctly but trailing dot incorrectly added).

The F format case already has the correct pattern — it guards with if (precision > 0) before adding the decimal part. The N format case lacked this guard.

Fix

Mirror the F format's guard:

case "n": case "N":
  precision = precision != null ? precision : 2;
  rep = toFixed(rep, precision);
  parts = splitIntAndDecimalPart(rep);
  if (precision > 0) {
    rep = thousandSeparate(parts.integral) + "." + padRight(parts.decimal, precision, "0");
  } else {
    rep = thousandSeparate(parts.integral);
  }
  break;

Test Plan

Added tests for N0, N2, and N (default) format specifiers via both ToString and String.Format:

• (1000).ToString("N0") → "1,000" ✓
• (12345678).ToString("N0") → "12,345,678" ✓
• (-1000).ToString("N0") → "-1,000" ✓
• (1000).ToString("N2") → "1,000.00" ✓
• (1000).ToString("N") → "1,000.00" ✓

Note: The P (percentage) and C (currency) format cases have the same trailing-dot issue for P0/C0. Those are lower-priority since their defaults are non-zero precision, but they could be fixed in a follow-up.

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