parser: add markdown doc about supported syntax

core/parser/SUPPORTED_FEATURES.MD

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+# SQL Parser Supported Features
+
+This document outlines the features currently supported by our work-in-progress SQL parser.
+
+## SELECT Statement
+
+- [x] Basic SELECT ... FROM ... structure
+- [x] Column selection
+  - [x] Star (*) selection
+  - [x] Individual column selection
+  - [x] Qualified column names (e.g., table.column)
+- [x] Table selection in FROM clause
+- [x] Table aliasing
+- [x] WHERE clause with conditions
+- [x] GROUP BY clause
+- [x] ORDER BY clause
+  - [x] ASC and DESC directives
+  - [x] Multiple column ordering
+- [x] LIMIT clause
+- [ ] OFFSET clause
+- [ ] HAVING clause
+- [ ] Subqueries
+
+## Operators and Expressions
+
+- [x] Basic arithmetic operators (+, -, *, /)
+- [x] Comparison operators (=, !=, <>, >, <, >=, <=)
+- [x] Logical operators (AND, OR, NOT)
+- [x] IN operator
+- [x] NOT IN operator
+- [x] LIKE operator
+- [x] BETWEEN operator
+- [x] Parenthesized expressions
+- [x] Function calls
+- [x] CASE expressions (both simple and searched)
+
+## JOINs
+
+- [x] INNER JOIN
+- [x] LEFT OUTER JOIN
+- [x] Multiple joins in a single query
+- [x] Join conditions (ON clause)
+- [ ] RIGHT OUTER JOIN
+- [ ] FULL OUTER JOIN
+- [ ] CROSS JOIN
+
+## Data Types and Literals
+
+- [x] String literals
+- [x] Numeric literals (integers and floats)
+- [ ] Date and time literals
+- [ ] Boolean literals
+- [ ] NULL
+
+## Functions
+
+- [x] Basic function calls with arguments
+- [x] Aggregate functions (SUM, AVG, etc.)
+- [ ] Window functions
+
+## Additional Features
+
+- [x] Case insensitivity for keywords
+- [x] Column aliasing
+- [ ] Common Table Expressions (CTEs)
+- [ ] Set operations (UNION, INTERSECT, EXCEPT)
+
+## Other SQL Statement Types
+
+- [ ] INSERT
+- [ ] UPDATE
+- [ ] DELETE
+- [ ] CREATE TABLE
+- [ ] ALTER TABLE
+- [ ] DROP TABLE
+- [ ] CREATE INDEX
+- [ ] CREATE VIEW