Fix global mappings and aesthetic aliases in validation

[georgestagg]

Currently the following fails validation:

$ ggsql validate 'SELECT x, y FROM data VISUALISE * DRAW point'
✗ Validation errors:
  - Layer 1: Layer 'point' mapping requires the aesthetic '`x`' (or '`y`').

similarly,

ggsql validate 'SELECT date, revenue, region FROM sales VISUALISE date AS x, revenue AS y DRAW line MAPPING region AS color' 
✗ Validation errors:
  - Layer 1: Layer 'line' does not support the `color` mapping. 

This PR ensures both global and local wildcard mappings are handled during validation, and also centralises where aesthetic aliases are defined, resolving them more generally and in methods like is_supported() (so that they can be used for validation).

After this change,

$ ggsql validate 'SELECT x, y FROM data VISUALISE * DRAW point'
✓ Query syntax is valid

Similarly,

$ ggsql validate 'SELECT date, revenue, region FROM sales VISUALISE date AS x, revenue AS y DRAW line MAPPING region AS color'
✓ Query syntax is valid