TSQL: Fix CASTing variables

[jpers36]

The TSQL dialect is unable to parse CAST statements which include parameters/variables. This fix will allow it to happen.

TSQL:
Add ParameterNameSegment to SingleIdentifierGrammar
Add CAST as a separate special function for parsing
Add CAST as an unreserved keyword
Adjust DECLARE syntax to remove Delimited logic that assumes statements have been delimited
Add test case handling the issue
Adjust test case YAMLs using CAST to represent that CAST is a keyword

[codecov]

Codecov Report

Merging #1627 (eca55c0) into main (05d172f) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main     #1627   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          131       131           
  Lines         9207      9211    +4     
=========================================
+ Hits          9207      9211    +4     

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

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