bug(expr): SUBSTR of unicode produces error: "byte index _ is not a char boundary"

We should not calculate by bytes, but by unicode character:

To reproduce:
Risingwave:

=> select substr('Mér', 1, 2);
SSL SYSCALL error: EOF detected

PSQL:

=> select substr('Mér', 1, 2);
 substr 
--------
 Mé
(1 row)

Similar for ''Mér'::char(3)

resources:

src/expr/src/vector_op/substr.rs:48:23

Line 37 in 95ab15c

    
           pub fn substr_start_for(s: &str, start: i32, count: i32, writer: &mut dyn Write) -> Result<()> {

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