Add "prefer library" rules

[jedrichards]

Disallow the usage of any document-local symbols, shared styles or shared text styles, preferring instead for these to come from libraries.

Define a rule option libraries which is an array of whitelisted libraries to act as sources. If the libraries array is empty then any library can be used as source. Investigate whether to use library names or ids in this option array.

Implement as three separate rules:

• symbols-prefer-library
• layer-styles-prefer-library
• text-styles-prefer-library