feat(NODE-7255): add well-known Symbol alias for _bsontype

[addaleax]

Make Symbol.for('@@mdb.bson.type') available as an alias for _bsontype. This simplifies downstream code by making it a reliable property of deserialized values that cannot conflict with _bsontype in user-provided (sub)documents, which would otherwise need to be detected through an additional prototype comparison.

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JS classes representing BSON values now have a bsonType symbol property as an alias for the existing _bsontype property. This makes it easier to distinguish between values of a specific BSON type and documents with arbitrary keys after deserializing:

import { bsonType, deserialize } from 'bson';

const doc = deserialize(...);
// will be set when doc.value is a BSON wrapper class **or** a sub-document { _bsontype: '...' } 
console.log(doc.value._bsontype); 
// will be set if and only if doc.value is a BSON wrapper class
console.log(doc.value[bsonType]);

It is safe to replace all uses of value._bsontype with value[bsonType].

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