RFR: 8366224: Introduce DecimalDigits.appendPair for efficient two-digit formatting and refactor DateTimeHelper [v3]

[Shaojin Wen]

> This PR introduces a new efficient API for appending two-digit integers to StringBuilders and refactors DateTimeHelper to leverage this new functionality.
> 
> Changes include:
> 
> 1. New `appendPair` method for efficient two-digit integer formatting (00-99):
>    - Added `AbstractStringBuilder.appendPair(int i)` with core implementation
>    - Added `JavaLangAccess.appendPair(StringBuilder, int)` for internal access
>    - Added `System.JavaLangAccessImpl.appendPair(StringBuilder, int)` bridge
>    - Added `DecimalDigits.appendPair(StringBuilder, int)` public static utility method
>    - Enhanced Javadoc documentation for all new methods
> 
> 2. Refactored `DateTimeHelper` to use the new `DecimalDigits.appendPair`:
>    - Updated `DateTimeHelper.formatTo` methods for `LocalDate` and `LocalTime`
>    - Replaced manual formatting logic with the new efficient two-digit appending
>    - Improved code clarity and consistency in date/time formatting
> 
> These changes improve code clarity and performance when formatting two-digit numbers, particularly in date/time formatting scenarios.

Shaojin Wen has updated the pull request incrementally with one additional commit since the last revision:

  Add DecimalDigitsTest to verify appendPair method with LATIN1 and UTF16 encoding

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Changes:
  - all: https://git.openjdk.org/jdk/pull/26911/files
  - new: https://git.openjdk.org/jdk/pull/26911/files/916db357..84ce1bdc

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26911&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26911&range=01-02

  Stats: 84 lines in 1 file changed: 84 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/26911.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26911/head:pull/26911

PR: https://git.openjdk.org/jdk/pull/26911