Postgres & T-SQL: Drop Function

[WittierDinosaur]

Brief summary of the change made

Adds Drop Function support to Postgres & T-SQL

Fixes #1908

Are there any other side effects of this change that we should be aware of?

No

Pull Request checklist

Tests & docstrings added

[WittierDinosaur]

Slightly mangled commit history from forgetting to pull from upstream 🤦‍♂️

[codecov]

Codecov Report

Merging #1924 (b491de3) into main (4d628b3) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main     #1924   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          143       143           
  Lines        10152     10160    +8     
=========================================
+ Hits         10152     10160    +8     

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

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[tunetheweb]

LGTM!