Adjust ANSI colors in logs and console for theme legibility (#10060)

* Adjust ANSI colors in logs and console for theme legibility

ANSI colors are fixed terminal colors, so some of them (e.g. ansi-black,
ansi-white, ansi-bright-white) are effectively invisible against the
corresponding light or dark themed background (see #6345).

textSpansFromAnsi now takes an optional background color, which all
console/logging call sites pass as the theme surface. Foreground colors
whose contrast ratio against that background is below 3:1 are blended
toward black (on a light background) or white (on a dark background) to
restore legibility; sufficiently contrasting colors are left untouched.
Background ANSI colors are not adjusted since blending into the surface
does not harm legibility.

Adds unit tests for the contrast adjustment behavior.

Fixes #6345

* Extract magic numbers in _adjustForContrast into named constants

Per code review feedback and the DevTools style guide.

* Add release note for ANSI color legibility improvements

* Remove unnecessary import from utils.dart

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Co-authored-by: awaisdev5765 <awaisdev5765@users.noreply.github.com>
Co-authored-by: Sam Rawlins <sam.rawlins@gmail.com>