feat: Support multiple values in SET

[MazterQyou]

b098e2d added ability to set several comma-separated variables, e.g. SET a = b, c = d. This however came with a regression that does not allow setting several comma-separated values for a variable, e.g. SET a = b, c.

One case where this is problematic is SET search_path = public, other_schema with PostgreSQL. Such a query is issued, for instance, by DBeaver.
This PR addresses the issue, allowing multiple values for a variable, instead of several variables (which is invalid syntax in Postgres) to be set when PostgreSQL or Redshift dialect is enabled.

Related test is included.

[coveralls]

Pull Request Test Coverage Report for Build 5959122578

• 35 of 41 (85.37%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 89.505%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/ast/mod.rs	1	3	33.33%
src/parser.rs	24	28	85.71%

Totals
Change from base Build 5027344687:	0.01%
Covered Lines:	8904
Relevant Lines:	9948

---

💛 - Coveralls