Search query fixup: simplify ParseTree type [3/3]

Created by: rvantonder

This stacked diff depends on #6115.

This is a semantics preserving change.

Summary

• Aliases ParseTree type to expression list and get rid of the struct and propagate the type.
• Simplifies tests, and it's now much clearer when doing make(syntax.ParseTree... instead of make([]*syntax.Expr...)

Test plan: This is a refactor, tests are updated.