Revise extension syntax

[ahouseholder]

This issue extends this comment:

This got me thinking: Maybe, we should make it easier for people to differentiate whether the extension segment is a language or an reverse DNS name (with optional fragment). I suggest to use ~ or @ or + as an indication that a reverse DNS segment starts.
Let's use the .. It is anyway in the DNS FQDN.

Originally posted by @tschmidtb51 in #821 (comment)

We need more details before we start adding more characters to the namespace spec. Specific usage examples that demonstrate the desired semantics will be helpful.

[ahouseholder]

This will be resolved as part of #858