css nesting : increase support

[romainmenke]

• handle nested at rules
• improve autocomplete and diagnostics when starting a nested rule
• improve parsing behaviour when a rule doesn't start with an ident

[romainmenke]

@aeschli friendly ping in case you missed these changes :)

[aeschli]

@romainmenke Oh sorry, I missed that. PR look good, thanks a lot!

[romainmenke]

Thank you @aeschli and @joyceerhl for reviewing this 🙇