T-SQL: ALTER FUNCTION/PROCEDURE/VIEW parsing

[differgroup]

Fixes the following T-SQL parsing error(s):
"Found unparsable section: 'ALTER FUNCTION...'
"Found unparsable section: 'ALTER PROCEDURE ...'
"Found unparsable section: 'ALTER VIEW ...'

The parsing for plain ALTER was missing.

The ALTER parsing is now implemented within the CreateFunctionStatementSegment(s),
as in T-SQL the syntax is otherwise similar.

Tests now also cover ALTER, as existing test cases have been modified to include all possible scenarios (CREATE, ALTER and CREATE OR ALTER).

[codecov]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (f3631d1) compared to base (4313db2).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #3867   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          179       179           
  Lines        13779     13779           
=========================================
  Hits         13779     13779           

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

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[greg-finley]

Thanks for the contribution!