Update scsslint.yml

[hnla]

BP has used a number of underscore separated tokens over time, sadly this is against convention and the scss linter defaults to selectortype: hyphenated_lowercase

Although we can avoid the use of these few token strings when they arise it can be a nuisance to especially when needing to override BP default styles that have used them.

I have added the Ignored_names line to the lint check block to use for these rare occurrences as we can't change or remove them from core templates safely and added the single use case I have at present.

[hnla]

In 9971:

Commit updates scss-lint.ylm to include the ignore_names line for SelectorFormat block.

This is to allow specific existing BP underscored selector name use.

• Add 'signup_form' id to list

Closes #6538