Feature: Add lightness option to modulate

[Tenpi]

This adds a lightness option to modulate in order to more closely mimic Photoshop's lightness adjustment. It does this by adding the provided value to the luminance channel instead of multiplying by it. Related to my earlier feature request (#2845).

Image Example:

Code Example:

await sharp(image)
      .modulate({lightness: 10})
      .toBuffer()

[coveralls]

Coverage decreased (-0.1%) to 99.9% when pulling 406bf5e on Tenpi:lightness into 7467fa8 on lovell:master.

[lovell]

Thank you very much for this, it looks great.

Is it worth adding a sentence to the JSDocs to clarify that lightness is additive whereas brightness is multiplicative?

Please can you add a unit test for the new error handling path too.

[Tenpi]

Please can you add a unit test for the new error handling path too.

Do you mean adding it to the "invalid options" test? If so, I have updated it.

[lovell]

Landed via commit 4a9267c, thank you. This will be in v0.29.1