TSQL: Finish all variants of expressions

[jimidle]

• Implement DEFAULT as a primitive expression
• Implement @localvar as a primitive expression
• Provide coverage test
• Removes defaulting to visiting terminal nodes

Closes: #275

[github-actions[bot]]

Coverage tests results

394 tests  ±0   354 :white_check_mark: +1   4s :stopwatch: ±0s   2 suites ±0     2 :zzz: ±0    2 files   ±0    38 :x:  - 1 

For more details on these failures, see this check.

Results for commit 226da0d6. ± Comparison against base commit f5836cfa.

:recycle: This comment has been updated with latest results.