Fix duckdb & sqlite character_length scalar unparsing

[Sevenannn]

Which issue does this PR close?

N/A

Rationale for this change

When unparsing CharacterLengthFunc scalar function to DuckDB & SQLite syntax SQL query, the CharacterLengthFunc scalar function need to be unparsed to length, since length is the equivalence of CharacterLengthFunc in DuckDB & SQLite.

DuckDB length() SQLite length()

What changes are included in this PR?

• Add struct DuckDBDialect, which implements the Dialect trait
• Add new method character_length_style for Dialect trait for determining the CharacterLengthStyle to use for a dialect
• Add helper function character_length_to_sql, which unparses the CharacterLengthFunc to length or character_length based on CharacterLengthStyle

Are these changes tested?

Yes

Are there any user-facing changes?

No

[alamb]

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