cli: use a palette to implement coloring
This basically replaces the (NMMetaTermColor, NMMetaTermFormat) combo with NMMetaColor that describes the colored element semantically as opposed to storing the raw attributes. A (currently static) paletted is used to translate the semantic color code to the actual ANSI controle sequence. This matches what terminal-colors.d(5) schemes use, making it convenient to implement customizable palettes.
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