Mirror `bat`s syntax / theme support

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• Match bat in gutter handling (ec91549)
• Use default background (63b6fa3)
• --list-themes support (a47ff5d)
• --list-languages support (60bdbce)
• Use bat's assets (ac111ec)
• Correctly handle ansi, base16, base16-256 themes
• Runtime asset loading See Loading of user themes / syntaxes #32

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Seeing if we can just depend on a subset of bat: sharkdp/bat#919 (comment)

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As for background color handling, they fill to the end of the line with whitespace to get the right background iff a background color is set.

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For the gutter (like line numbers), bat uses

const DEFAULT_GUTTER_COLOR: u8 = 238;

       let gutter_style = Style {
            foreground: match theme.settings.gutter_foreground {
                // If the theme provides a gutter foreground color, use it.
                // Note: It might be the special value #00000001, in which case
                // to_ansi_color returns None and we use an empty Style
                // (resulting in the terminal's default foreground color).
                Some(c) => to_ansi_color(c, true_color),
                // Otherwise, use a specific fallback color.
                None => Some(Fixed(DEFAULT_GUTTER_COLOR)),
            },
            ..Style::default()
        };