Consider providing separate `bold` and `bright` modifiers for custom colors

**Is your feature request related to a problem? Please describe.**

In Solarized, the "bright" colors (ANSI colors 8 to 15) are used for separate hues not always related to their regular (ANSI colors 0-7) counterparts.

In lazygit, it is impossible to specify these hues without also making the text bold. Likewise, it is impossible to specify these hues in background context (the `bold` attribute is ignored in background context which makes sense because no "bold background" attribute is defined in the SGR language).

**Describe the solution you'd like**

I'd like lazygit to implement a separate `bright` color modifier whose effect would be to add `60` to the corresponding SGR color attribute _without_ also emitting the `1;` bold attribute.

Please note that this modifier should be applicable in both foreground (emitting attributes 90 to 97) and background (emitting attributes 100 to 107) contexts.

**Describe alternatives you've considered**

- not using Solarized
- not using ANSI colors 8 to 15

**Additional context**

See [`console_codes`(4)](https://man.archlinux.org/man/console_codes.4#Linux_console_controls) under the "ECMA-48 Select Graphic Rendition" subheading.