Add infrastructure for linting and creating new Lints

[orpuente-MS]

This PR adds infrastructure for linting and creating new Lints. It also adds integration of the linter to the language service.

End user experience

The users can set the lint level from the qsharp.json file in a project as follows,

{
  "lints": [
    {
      "lint": "needlessParens",
      "level": "allow",
    },
    {
      "lint": "redundantSemicolons",
      "level": "warn",
    },
    {
      "lint": "divisionByZero",
      "level": "error",
    }
  ]
}

Contributor experience

The entry points to the linter is the run_lints function, which takes a qsc_frontend::CompileUnit as input and outputs a Vec<Lint>.

Example

use linter::run_lints;;
use qsc::compile::compile;

let unit: CompileUnit = compile(...);

// The second argument is an optional user configuration.
let lints: Vec<Lint> = run_ast_lints(&package, None);

How to add a new Lint

We can add a new lint in two steps:

1. Declaring the lint: here we set the lint name, the default LintLevel, the message, and the help text the user will see.
2. Implementing the lint: here we write the pattern matching logic of the new lint.

Below is a full example of how to add a new AST lint.

Example

First, we add our lint to src/lints/ast.rs.

declare_ast_lints! {
  ...
  (DoubleParens, LintLevel::Warn, "unnecesary double parentheses", "remove extra parentheses for clarity"),
}

Then we implement the right LintPass for our new lint, in this case linter::ast::AstLintPass

impl linter::ast::AstLintPass for DoubleParens {
    // we only need to impl the relevant check_* method, all the other ones
    // will default to an empty method that will get optmized by rust
    fn check_expr(&self, expr: &qsc_ast::ast::Expr, buffer: &mut Vec<Lint>) {
        // we match the relevant pattern
        if let ExprKind::Paren(ref inner_expr) = *expr.kind {
            if matches!(*inner_expr.kind, ExprKind::Paren(_)) {
                // we push the lint to the buffer
                push_lint!(self, expr.span, buffer);
            }
        }
    }
}