[MINOR][SQL] Put the grammar of database together, because this is good for maintenance and readability.

## What changes were proposed in this pull request?

The SQL grammar `SHOW DATABASES` is mixed in some grammar of table. I think should arrange the grammar of database together.
This is good for maintenance and readability.

## How was this patch tested?

No UT

Closes apache#24138 from beliefer/arrange-sql-grammar.

Authored-by: gengjiaan <[email protected]>
Signed-off-by: Sean Owen <[email protected]>

Author: beliefer

sql/catalyst/src/main/antlr4/org/apache/spark/sql/catalyst/parser/SqlBase.g4

@@ -83,6 +83,7 @@ statement
         (WITH DBPROPERTIES tablePropertyList)?                         #createDatabase
     | ALTER database identifier SET DBPROPERTIES tablePropertyList     #setDatabaseProperties
     | DROP database (IF EXISTS)? identifier (RESTRICT | CASCADE)?      #dropDatabase
+    | SHOW DATABASES (LIKE? pattern=STRING)?                           #showDatabases
     | createTableHeader ('(' colTypeList ')')? tableProvider
         ((OPTIONS options=tablePropertyList) |
         (PARTITIONED BY partitionColumnNames=identifierList) |
@@ -153,7 +154,6 @@ statement
         (LIKE? pattern=STRING)?                                        #showTables
     | SHOW TABLE EXTENDED ((FROM | IN) db=identifier)?
         LIKE pattern=STRING partitionSpec?                             #showTable
-    | SHOW DATABASES (LIKE? pattern=STRING)?                           #showDatabases
     | SHOW TBLPROPERTIES table=tableIdentifier
         ('(' key=tablePropertyKey ')')?                                #showTblProperties
     | SHOW COLUMNS (FROM | IN) tableIdentifier