fix StringUtils.getDigits dropping supplementary digits

[alhudz]

1. getDigits scans the input one char at a time and calls Character.isDigit(char), so a supplementary digit such as U+1D7CF (MATHEMATICAL BOLD DIGIT ONE) is dropped: neither surrogate half is itself a digit, even though Character.isDigit reports the code point as a digit.
2. Switched the scan to code points (codePointAt/charCount) and write the matched code point back with Character.toChars.

Repro getDigits("a" + new String(Character.toChars(0x1D7CF)) + "9"): expected the supplementary digit then 9, actual 9. BMP input (including the existing Devanagari case) is unchanged.

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