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: vim#15311

Signed-off-by: Yinzuo Jiang <[email protected]>
Signed-off-by: Christian Brabandt <[email protected]>