Implement SQL String functions with documentations

[AmrDeveloper]

Implement string function in our engine, one per pull request.

Steps:

• Choose unimplemented function and write I am on <function_name>.
• Then start implement the function in crates/gitql-ast/function.rs.
• Add docs for it in docs/function/function.md.
• Make sure everything is formatted and submit a Pull Request.
• Don't forget to mention issue number in pull request title.

Text Functions

• [x] ASCII
• [x] CHAR
• [x] CHARINDEX
• [x] CONCAT
• [x] DATALENGTH
• [ ] DIFFERENCE
• [ ] FORMAT
• [x] LEFT
• [x] LEN
• [x] LOWER
• [x] LTRIM
• [x] NCHAR
• [ ] PATINDEX
• [x] QUOTENAME
• [x] REPLACE
• [x] REPLICATE
• [x] REVERSE
• [x] RIGHT
• [x] RTRIM
• [x] SOUNDEX
• [x] SPACE
• [ ] STR
• [x] STUFF
• [x] SUBSTRING
• [x] TRANSLATE
• [x] TRIM
• [x] UNICODE
• [x] UPPER

For more information about implementation and description check https://www.w3schools.com/sql/sql_ref_sqlserver.asp

Note: You can only open max 3 PR at the time