Types: add "linear" to KernelEnum

[BayanBennett]

This matches the JS code

sharp/lib/resize.js

Lines 69 to 78 in 8607ff2

	const kernel = {
	nearest: 'nearest',
	linear: 'linear',
	cubic: 'cubic',
	mitchell: 'mitchell',
	lanczos2: 'lanczos2',
	lanczos3: 'lanczos3',
	mks2013: 'mks2013',
	mks2021: 'mks2021'
	};

[lovell]

Thank you!