dartdoc should allow nnbd ? and ! inside comment references

[jcollins-g]

I expect people will write things like [Object?] and possibly even [fieldName!] to refer to elements in comment references. Support this similarly to how we used to strip out 'new' and other extraneous information.

[mit-mit]

@jcollins-g how is this progressing?

[devoncarew]

cc @srawlins

[srawlins]

I can prioritize this for September.

[srawlins]

Fixed by #2328