Introduce MapKeyNode interface to limit node types for map key

[itchyny]

This pull request introduces ast.MapKeyNode interface for the following reasons.

• There are only limited node types for map keys. For example MappingValueNode never allowed as a map key. But current AST allows to create such unexpected maps. This PR changes the argument key of ast.MappingValue to ast.MapKeyNode to prohibit creating unexpected maps.
• In the ast package, stringWithoutComment() string is implemented for all node types, but this method is only required for map key types. CommentNode having stringWithoutComment looks very weird. So I removed all the unnecessary stringWithoutComment implementations.

For ast package users, following is the complete list of the changes.

• New interface ast.MapKeyNode is introduced. All the node types implementing ast.ScalarNode and ast.MergeKeyNode, ast.MappingKeyNode satisfy ast.MapKeyNode. Note that ast.LiteralNode never appears as map key but is a scalar node thus satisfies ast.MapKeyNode.
• ast.Null, ast.Bool, ast.Integer, ast.Float now return the concrete types instead of ast.Node (Ref: The implementing package should return concrete (usually pointer or struct) types).
• ast.MappingValue now accepts key ast.MapKeyNode instead of ast.Node. Also, func (*MapNodeIter) Key() now returns ast.MapKeyNode instead of ast.Node.

[goccy]

Thank you for the great PR !!! ast.MapKeyNode is very nice ! LGTM 😄