refactor: add new `DocSheet` as middle layer for hierarchical file parsing

[wenchy]

refer Pandas: tabular data

https://github.com/tableauio/tableau/blob/68ffb131604d8e74f8e3bc4d69d1f14ed6fd6b95/internal/importer/book/sheet.go#L35-L43

Support two kinds of descriptors:

• [x] flat formats: XLSX/CSV -> FlatSheet
• [ ] hierachy formats: XML/YAML -> DocSheet

Define FlatSheet and DocSheet

 type baseSheet struct {
    Name string
    Meta * tableaupb.Metasheet
}

type FlatSheet struct {
    baseSheet
    Rows[][] string // 2D array of strings. 
}

type Attribute struct {
    Name string
    Value string
}

type Node struct {
    Name string
    Attributes[] Attribute
    Children []*Node
}

// NOTE: descriptor sheet (name starts with "@") describes the corresponding sheet's structure.
// E.g: "@ItemConf" describes the structure of "ItemConf"
type DocSheet struct {
    baseSheet
    Nodes[] Node
}

Refer:

• Go XML Node
• FileDescriptorSet