### What changes were proposed in this pull request?

Checking wether variable declaration is only at the beginning of the BEGIN END block.

### Why are the changes needed?

SQL standard states that the variables can be declared only immediately after BEGIN.

### Does this PR introduce _any_ user-facing change?

Users will get an error if they try to declare variable in the scope that is not started with BEGIN and ended with END or if the declarations are not immediately after BEGIN.

### How was this patch tested?

Tests are in SqlScriptingParserSuite. There are 2 tests for now, if declarations are correctly written and if declarations are not written immediately after BEGIN. There is a TODO to write the test if declaration is located in the scope that is not BEGIN END.

### Was this patch authored or co-authored using generative AI tooling?

No

Closes #47404 from momcilomrk-db/check_variable_declarations.

Authored-by: Momcilo Mrkaic <momcilo.mrkaic@databricks.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>