Add support for exporting colors from variables

[alexey1312]

• Added ColorsVariablesLoader
• Updated doc

Scheme

From the documentation:

1. tokensCollectionName - the name of the variable collection
2. lightModeName - the name of the color variable column for the light theme
3. darkModeName - the name of the color variable column for the dark theme
4. lightHCModeName - the name of the color variable column for the light theme with high contrast
5. darkHCModeName - the name of the color variable column for the dark theme with high contrast
6. A variable that has a local value
7. A variable that refers to another variable in a different file

1. primitivesModeName - the name of the variable column, if the value in CONFIG.md is not specified, the default value will be used
2. A variable that has a local value

fyi:
#217