New lint: comment line density for files and functions

What it does

The lint counts the number of lines of the file and count the number of comments in the source file and do a percentage
if the percentage is bellow a specific threshold a warning is raised

In tools like sonarqube it corresponds to comment_lines_density https://docs.sonarsource.com/sonarqube/latest/user-guide/code-metrics/metrics-definition/

Comment Density (%) = (Number of Comment Lines / (Total Lines - Empty Lines)) × 100

Advantage

It is important for some industries (like space industry), some coding standards request it

Drawbacks

This metric is generally not relevant and we want developers to think about relevant comments instead of number of lines of comments, but as said in Advantage it is something requested by standards

Example

<code>
// Calculate nth Fibonacci number
fn fibonacci(n: u32) -> u32 {
    if n <= 1 {
        return n;
    }

    let mut a = 0;
    let mut b = 1;

    for _ in 2..=n {
        let next = a + b;
        a = b;
        b = next;
    }

    b
}

fn main() {
    // Test Fibonacci
    let n = 10;
    println!("Fibonacci of {} is {}", n, fibonacci(n));
}

Could be written as with 52 % (13 of 25) of comments:

<code>
// This function calculates the nth Fibonacci number
fn fibonacci(n: u32) -> u32 {
    // Base case: If n is 0 or 1, return n
    if n <= 1 {
        return n;
    }

    // Initialize variables for the iterative calculation
    let mut a = 0; // This will hold the (n-2)th Fibonacci number
    let mut b = 1; // This will hold the (n-1)th Fibonacci number

    // Loop from 2 to n to calculate the nth Fibonacci number
    for _ in 2..=n {
        // Calculate the next Fibonacci number in the sequence
        let next = a + b;

        // Update a and b for the next iteration
        a = b; // Shift b to a
        b = next; // Set b to the newly calculated Fibonacci number
    }

    // Return the nth Fibonacci number
    b
}

fn main() {
    // Example usage: Calculate and print the 10th Fibonacci number
    let n = 10; // Change this value to calculate a different Fibonacci number
    println!("Fibonacci of {} is {}", n, fibonacci(n));
}

No implementation files, tests, or entry points are named. Start by clarifying whether the lint must measure whole files, individual functions, or both, and how Rust comment forms and empty lines are counted. Done should include an agreed threshold configuration, warnings for below-threshold density, and coverage for the Fibonacci examples.