J-P Nurmi 42b7de838e [Linux][a11y] implement AtkText::get_text/string_at_offset() (flutter/engine#38144)
This PR implements `AtkText::get_string_at_offset()` (and the deprecated `AtkText::get_text_at_offset()` still used by e.g. Orca) for `FlAccessibleTextField` to allow Orca to read out loud the current character while moving the text cursor around.

### Before (unmute to hear the screen reader)

[textfield-a11y-before.webm](https://user-images.githubusercontent.com/140617/206556644-fb4f4df8-acca-4d97-86d5-7120f0a4871d.webm)

### After (unmute to hear the screen reader)

[textfield-a11y-after.webm](https://user-images.githubusercontent.com/140617/206556678-4fbf9112-291e-4518-a258-e9ca33469430.webm)

Fixes: flutter/flutter#113049

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
2023-06-07 17:18:04 +00:00
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