Rebrand to `atanor`

[paolobrasolin]

Given the naming conflict with Typst's package fletcher, we need a new name so we might as well improve on it.

The new name is, of course, atanor:

• nigredo (coerce ∘ parse): putrefy code into source AST
• albedo (coerce ∘ inject): purify source AST into UL
• citrinitas (project): transmute UL into target AST
• rubedo (render): coagulate target AST into code

[paolobrasolin]

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