Add Pandoc syntax for inline roles

[hason]

For roles it would be great to support pandoc syntax (try):

[interpreted text]{role=}
[interpreted text]{role=custom}

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[choldgraf]

Hey there @hason - for requests to extend / change MyST syntax, it will be important to provide rationale for why a new syntax should be used, references to other places that it is used (e.g. in the pandoc documentation), reference implementations etc. Note that you can also write your own markdown-it-py extensions to support new syntax you'd like to write in. As a general rule, I don't think we want to casually add new syntactic pieces to MyST...it should only be done if there is really a strong need and after community conversation (though in general, it should be possible for people to write their own personal syntax extensions)

(the same applies to #245)