Do not migrate variant = 'outline' during upgrades

[RobinMalfait]

This PR improves the upgrade tool for shadcn/ui projects where the variant = "outline" is incorrectly migrated to variant = "outline-solid".

This PR also handles a few more cases:

// As default argument
function Button({ variant = "outline", ...props }: ButtonProps) { }

// With different kinds of quotes (single, double, backticks)
function Button({ variant = 'outline', ...props }: ButtonProps) { }

// Regardless of whitespace
function Button({ variant="outline", ...props }: ButtonProps) { }

// In JSX
<Button variant="outline" />

// With different quotes and using JavaScript expressions
<Button variant={'outline'} />

// As an object property
buttonVariants({ variant: "outline" })

[philipp-spiess]

FINE