[io] support rgb colours for ansi escape codes

[Lexedia]

This PR adds RGB colour support for ANSI escape codes.
I wasn't sure how to approach this, so any suggestion is welcome.

Also for the changelog, I have no clue if it's the right way to do it considering 1.1.0 never got released.

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[gemini-code-assist]

Code Review

The pull request introduces RGB color support for ANSI escape codes, which is a valuable addition. The AnsiRgbCode class and rgb utility function are well-documented. The example and tests demonstrate the new functionality effectively. However, there are a few areas for improvement regarding robustness, correctness, and adherence to Dart's type system. Specifically, the _gradient function needs to handle single-character strings gracefully, the AnsiRgbCode class has a redundant and problematic field override, and the wrapWith function should prevent combining standard and RGB color codes of the same type. There are also minor issues with a typo in a test description and an error message.

[gemini-code-assist]

The _gradient function can produce NaN for ratio when text contains a single character (e.g., length is 1), leading to a runtime error due to division by zero. This can be fixed by handling the single-character case explicitly to prevent this issue, improving the robustness of the function.

Suggested change

	void _gradient(String text, bool forScript) {
	final length = text.length;
	final buffer = StringBuffer();
	for (var i = 0; i < length; i++) {
	final ratio = i / (length - 1);
	int red, green, blue;
	if (ratio < .5) {
	red = ((1 - (ratio * 2)) * 255).round();
	green = (ratio * 2 * 255).round();
	blue = 0;
	} else {
	red = 0;
	green = ((1 - ((ratio - .5) * 2)) * 255).round();
	blue = (((ratio - .5) * 2) * 255).round();
	}
	buffer.write(rgb(red, green, blue).wrap(text[i], forScript: forScript));
	}
	print(buffer.toString());
	void _gradient(String text, bool forScript) {
	final length = text.length;
	if (length == 0) return;
	final buffer = StringBuffer();
	for (var i = 0; i < length; i++) {
	final ratio = length == 1 ? 0.5 : i / (length - 1);
	int red, green, blue;
	if (ratio < .5) {
	red = ((1 - (ratio * 2)) * 255).round();
	green = (ratio * 2 * 255).round();
	blue = 0;
	} else {
	red = 0;
	green = ((1 - ((ratio - .5) * 2)) * 255).round();
	blue = (((ratio - .5) * 2) * 255).round();
	}
	buffer.write(rgb(red, green, blue).wrap(text[i], forScript: forScript));
	}
	print(buffer.toString());
	}

[gemini-code-assist]

The wrapWith function currently allows both a standard foreground/background AnsiCode and an AnsiRgbCode of the same type to be present in the codes iterable. This can lead to ambiguous or incorrect ANSI escape sequences (e.g., ESC[34;38;2;255;0;0m). The foreground and background counters should also account for AnsiRgbCode instances to ensure only one color type is applied per foreground/background. This improves correctness and maintainability.

    if (code is AnsiRgbCode) {
      if (code.type == AnsiCodeType.foreground) {
        foreground++;
      } else if (code.type == AnsiCodeType.background) {
        background++;
      }
      ansiRgbCode = code;
    } else if (code.code != -1) {
      numericCodes.add(code.code);
    }