Add PostgreSQL language support

[farhad-sadeghlo]

Summary

Adds PostgreSQL as a supported language in the VS Code LeetCode extension.

Changes

• Adds postgresql to the supported language list.
• Maps PostgreSQL solutions to the .sql file extension.
• Adds PostgreSQL to the leetcode.defaultLanguage configuration.
• Adds PostgreSQL-specific file path configuration.
• Maps PostgreSQL Markdown code blocks to SQL syntax highlighting.
• Updates the supported-language documentation.

Validation

• npm run compile
• npm run lint
• npm run build
• Generated a PostgreSQL solution for LeetCode problem 175 using the language slug postgresql.

The generated file contained:

@lc app=leetcode id=175 lang=postgresql

and:

Write your PostgreSQL query statement below

Dependency

This feature depends on PostgreSQL support in the underlying vsc-leetcode-cli package. This pull request should remain a draft until those changes are implemented and a new package version is published.