Report a dictionary value by its key

[tgharold]

Stacked on #91. Retarget to master after #91 merges; CI runs only for PRs into master.

Motivation

A dictionary value was reported by its position in enumeration order, through the KeyValuePair the dictionary yields: Map[0].Value.Name. A caller can't map that back to a key without enumerating the dictionary the same way. For 3.0 the ruling is to report by key, in the form MVC model binding uses for a dictionary (selectedCourses[1050]=Chemistry): Map[Primary].Name. Keys stay unquoted and unescaped. An object key keeps the index form for its entry, so the key is still validated.

Summary

• EnqueueItems unwraps the entries of a dictionary, meaning any type that implements IDictionary, IDictionary<,> or IReadOnlyDictionary<,>. When the key is simple, the value is queued at [key]. A simple key is a string, primitive, enum or self-formatting struct that is a leaf type, formatted with the invariant culture. Any other entry keeps [index].Key / [index].Value. A List<KeyValuePair<,>> is not a dictionary.
• The value is one level below the dictionary, like a list item, so it can come earlier in the results and counts one level less for the depth limit.
• The struct-skip from Report a struct once when a collection repeats it #91 now checks what is queued. A dictionary of class values re-enumerated by a second route no longer skips an entry that a Validate method added, which Report a struct once when a collection repeats it #91 alone would have done, because KeyValuePair is a struct.
• PathStep gains a key step. Enqueue takes a ready-made step. The old NOTE about dictionaries is resolved.
• BREAKING. Existing tests move to the key form. The 2.x dictionary test leaves UpgradeCompatibilityTests. README gets a Dictionaries section, and the changelog gets a BREAKING entry.

Test plan

• Specs first: 27 specs fail on the Report a struct once when a collection repeats it #91 branch, and the guards pass (object key, key with an attribute, null value, list of pairs)
• With the fix: 637 passed, 7 skipped on net8.0 and net10.0
• Test project builds for net481 with -p:IncludeNetFramework=true
• CI after retargeting to master