[Enhancement] Extension scopes

[iamgio]

Before submitting

• I am using the latest version of Quarkdown.
• I have checked the wiki to confirm this feature doesn't already exist.
• I have searched existing issues for duplicates.

Description

Should allow .extend in scopes to only act within that scope, including primitive extensions.

Proposed syntax or behavior

.if {yes}
    .extend {heading}
        .super foreground:{blue}

    # Blue

# Not blue

Additional context

No response