fix(ecmascript): trim trailing whitespace in string to number (#26148)

## Summary

`StringToNumber` previously removed ECMAScript whitespace only from the beginning of a string. Legal trailing whitespace therefore remained in the value passed to Rust’s numeric parsers, causing valid JavaScript numeric strings to produce `NaN`.

For example:

```js
"1 " * 2       // incorrectly folded to NaN
"0x10 " - 0    // incorrectly folded to NaN
```

This changes `trim_start_matches` to `trim_matches`, applying the existing ECMAScript whitespace predicate to both ends of the string.

After the change:

```js
"1 " * 2       // folds to 2
"0x10 " - 0    // folds to 16
```

The custom predicate remains important because ECMAScript and Rust define whitespace differently. Tests cover every ECMAScript whitespace character and verify that non-ECMAScript whitespace such as U+0085 is not accepted.