docs: add about scalar/exists/in subqueries to sql-features.md

[ashigeru]

This pull request updates the SQL feature documentation to add new feature - scalar subqueries, exists predicate, in predicate with subqueries. And clarify their current limitations and add details about scalar subqueries.
It also adjusts the planned features section to better reflect future development priorities.

Enhancements and clarifications to SQL feature documentation:

• Added a new section describing scalar subqueries, including syntax and limitations such as lack of support for correlated subqueries and restrictions on their use in join conditions.
• Documented limitations for IN with subqueries, specifying its restricted usage in filter clauses and noting that NOT IN with subqueries is not supported in this version.
• Added limitations for the EXISTS predicate, referencing the same restrictions as scalar subqueries.

Updates to feature listings:

• Updated the feature list and grammar to include scalar subqueries and IN with query expressions. [1] [2] [3]
• Revised the planned features section to clarify that correlated subqueries and full support for IN/NOT IN with subqueries are future goals.

General wording improvement:

• Improved the reference to the planned features section for clarity.