MyBatis Added SQL_SERVER_SNAPSHOT value to TransactionIsolationLevel enum

Sql Server supports a non-standard transaction isolation level called "SNAPSHOT" whose JDBC value is 0x1000 (4096). The new enum value adds support for this isolation level.

The name "SQL_SERVER_SNAPSHOT" was chosen to reflect the fact that it is unique to SQL Server to avoid confusion. See #1969

Comment From: harawata

@maltalex , The new constant should be available in the latest 3.5.6-SNAPSHOT. Please let me know if there is any issue. Thank you again for the report and this PR!