compare and convert system types properly

[jycor]

Changes:

• IsType() methods now account for system variable types,
• coalesce type comparison accounts for system variable types
• create table from select ... statements account for system variable types

Note: MySQL lists @@admin_port as Integer in their docs, but shows up as UInteger in the CLI

fixes: dolthub/dolt#8448

[max-hoffman]

LGTM. The extension to this might be adding tests for other functions, presumably we don't test system types in many other places.

[jycor]

regression test fix here: dolthub/doltgresql#856