Add initial Redshift support

[tunetheweb]

Brief summary of the change made

Adds initial Redshift support as had a few queries on it.
It inherits form Postgres but gets:

• It's own keyword list as requested in AWS Redshift Dialect support #612 (comment)
• Support for IGNORE NULL as requested in ignore null causes parsing error for Redshift #1629
• Support for DATEDIFF as requested in TSQL - DATEDIFF() function first argument not parsed as a keyword #1623
  The main reason for adding is to give us a basis from which to expand.

Fixes #612
Fixes #1629
Progress on #1623

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

Based off of Postgres for now as, according to Wikipedia:

Amazon Redshift is based on an older version of PostgreSQL 8.0.2, and Redshift has made changes to that version.
Plus most redshift users have been using that dialect up to now.
Will need to try our best to manage any differences (ideally just override in Redshift dialect rather than introduce too much non-Postgres stuff to that dialect).

Pull Request checklist

• Please confirm you have completed any of the necessary steps below.

• Included test cases to demonstrate any code changes, which may be one or more of the following:

  • .yml rule test cases in test/fixtures/rules/std_rule_cases.
  • .sql/.yml parser test cases in test/fixtures/dialects (note YML files can be auto generated with python test/generate_parse_fixture_yml.py or by running tox locally).
  • Full autofix test cases in test/fixtures/linter/autofix.
  • Other.

• Added appropriate documentation for the change.

• Created GitHub issues for any relevant followup/future enhancements if appropriate.

[codecov]

Codecov Report

Merging #1641 (95a3bba) into main (ed23bf6) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main     #1641   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          131       133    +2     
  Lines         9223      9243   +20     
=========================================
+ Hits          9223      9243   +20     

Impacted Files	Coverage Δ
src/sqlfluff/core/dialects/__init__.py	100.00% <ø> (ø)
src/sqlfluff/dialects/dialect_redshift.py	100.00% <100.00%> (ø)
src/sqlfluff/dialects/dialect_redshift_keywords.py	100.00% <100.00%> (ø)

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[WittierDinosaur]

Looks good to me! LGTM