Implement source code environment variable detection

[suriyasureshok]

Scan source files and extract referenced environment variables.

Initial Support:

• Rust
• Python
• JavaScript
• TypeScript

Examples:

Rust:
env::var("DATABASE_URL")

Python:
os.getenv("DATABASE_URL")

JavaScript:
process.env.DATABASE_URL

Acceptance Criteria:

• Extract variable names
• Return unique variables
• Ignore duplicates

[sudarsan1030]

hiii , i would like to work on this issue