runtime(mysql): update syntax script

runtime(mysql): update syntax script

Problem:

- `syn region ...`s in syntax/mysql.vim match function names inaccurately.
- no syntax rules for mysql window function.
- coarse highlight definition in syntax/mysql.vim.

Solution:

- add `\<` before the function name for accuracy.
- add syntax rules for mysql window function.
- enhance the highlight definition.

closes: #15311

Signed-off-by: Yinzuo Jiang <jiangyinzuo@foxmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>