Fix DECLARE Delimitation

[jpers36]

Updating the TSQL DECLARE statement to properly handle optional delimitation, similar to the changes in #1612 .

[codecov]

Codecov Report

Merging #1615 (89cd067) into main (2a72241) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main     #1615   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          131       131           
  Lines         9208      9207    -1     
=========================================
- Hits          9208      9207    -1     

Impacted Files	Coverage Δ
src/sqlfluff/dialects/dialect_tsql.py	100.00% <100.00%> (ø)

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