Optional default tag for union types

[dahlia]

If union types can set its default tag it would help backward compatibility e.g.:

record name (text fullname);
// The below record type can be migrated to the above union type:

// Pseudo code — syntax is just for example
union name
    = wastern-name (text first-name, text? middle-name, text last-name)
    | east-asian-name (text family-name, text given-name)
    | default culture-agnostice-name (text fullname)
    ;

The default tag is implicitly determined if the JSON object doesn’t have "_tag" field, which means the case a record type is refactored to a union type.

• [x] Add default keyword. (#227)
• [x] Write section about default into docs/refactoring.md.

[dahlia]

Remaining things after PR #227:

• [x] Register a new keyword default to Nirum.Constructs.Identifier.reservedKeywords set.
• [x] Refactor duplicated codes to union tags and defaultTag.
• [x] Remove __valerie__ = True.
• [x] Write section about default into docs/refactoring.md.