[TASK] Fluid ViewHelper: Add sizes option

Similar to the Fusion prototype, allow setting a sizes attribute that
defines which size variants are generated (instead of the default
set taken from Settings).

The sizes are passed as a comma-separated string.

[TASK] Make installable in Neos 9

[TASK] Make installable in Neos 8

[TASK] Set minimum dependency to Neos 5.x

DOCS: Smaller adjustments in the docs (#5)

* TASK: Add composer install instruction

* TASK: Adjust variable name to prevent confusion

The fusion variable name imageUri lead to confusion, because you will get a image
tag and not just an uri.

* TASK: Change site package to upper case

* TASK: Small languages fixes

[TASK] Make installable in Neos 3.0+

Merge pull request #2 from jonnitto/patch-1

Make package ready for Neos 4.*

[BUGFIX] Allow ratio null in ViewHelper

[TASK] Keep original CSS classes in ViewHelper

[FEATURE] Add ImageViewHelper