[V1 format] decide how to specify/resolve entry reference in the lexicon csv

[hayashi-mas-wap]

relates to #353.

In V1 format, words in lexicon csv can be referred using triple ref (set of headword, pos, reading).
However, this reference is ambiguous (there can be a multiple entry that match the ref).

Questions:

• How granular does the reference should be (is it enough to be current triple or we need other columns)?
• Which entry should be prioritized when a reference matches multiple entries?

[hayashi-mas-wap]

We will add another column to lexicon csv: reference-id.

• this column is optional, and the value type is optional[str]
• the value is used to distinguish entries with same headword-pos-reading set
  • the values must be unique in a lexicon csv
• we expand current triplet reference to headword, pos, reading[, reference-id]
• this information is used only for building binary dictionary
  • system dictionary binary will keep them for user dictionary build.