support reading of properties on implicit block mapping keys

[dota17]

now , the following yaml document can be loaded

&anchor foo:
  foo: bar
  *anchor: duplicate key
  baz: bat
  *anchor: duplicate key

[perlpunk]

Note that the example is invalid YAML.
There needs to be a space after an alias key before the colon (because a colon can be part of an anchor/alias).
It should be:

  *anchor : duplicate key

[rlidwka]

pyyaml parses this, however http://ben-kiki.org/ypaste/ (which is specified as reference parser) does not.

Gonna check it again later, but I think we should require a space after colon there?

[perlpunk]

pyyaml and libyaml can parse it, but that is actually a bug.

[yuriy-yarosh]

And what about object / array anchor merging ?
Is it a valid syntax for Yaml 1.2 ?

"@figen/config-eslint@*":
  dependencies: &eslint
    "eslint": "8.55.0"
    
"@figen/config-typescript@*":
  dependencies: &typescript
    "typescript": "^5.3.3"
      
"figen@*":
  dependencies:
    << : {*eslint, *typescript}

[perlpunk]

Correct would be:

<< : [*eslint, *typescript]

The syntax itself is correct. The parser doesn't know anything about << - it is just a string at this point.
The constructor detects it as a merge key, and that depends on library if it allows this for YAML 1.2. It should not be enabled by default.