Move sigil into source of UnsupportedAnnotation

[stasm]

Follow-up to #643.

IIUC, the data model stability policy now says that if we ever un-reserve, say, the % sigil, we will also need to keep it on the list of sigils inside UnsupportedAnnotation:

interface UnsupportedAnnotation {
  type: "unsupported-annotation";
  sigil: "!" | "%" | "^" | "&" | "*" | "+" | "<" | ">" | "?" | "~";
  source: string;
}

Otherwise, (i.e. if we remove % from the list), data model representations produced by older implementations will not be valid.

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What was the reason for including the sigil field in UnsupportedAnnotation? Could we move the sigil into the source?

Originally posted by @stasm in #643 (comment)