Expected 1 space after colon with &:not(:first-child)

### Preconditions

1. In a less file if you use &:not(:first-child) it generates spacing warnings.

### Steps to reproduce

1. Use &:not(:first-child) in a less file.
2. Run the coding standard from command line. `vendor/bin/phpcs --standard=Magento2 path/to/module`

### Expected result

1. No Warnings are generated.

### Actual result

1. Warnings are generated
154 | WARNING | Expected 1 space after colon in style definition; newline found
154 | WARNING | Expected 1 space after colon in style definition; 0 found

Reproduce the warning with a LESS file containing `&:not(:first-child)` and run `vendor/bin/phpcs --standard=Magento2 path/to/module`. Start by locating the Magento2 rule that checks spacing in style definitions; done means the command produces no warnings for this selector.

Written by the indexing model from the issue text.