Refactor the manifest code to contain all modification

[ruben-arts]

As an example this function works both on the toml table and the the actual manifest code. All toml modification should be located in the manifest code.

pub fn add_task(
        &mut self,
        name: impl AsRef<str>,
        task: Task,
        platform: Option<Platform>,
    ) -> miette::Result<()> {
        let table = if let Some(platform) = platform {
            if self
                .target_specific_tasks(platform)
                .contains_key(name.as_ref())
            {
                miette::bail!("task {} already exists", name.as_ref());
            }
            ensure_toml_target_table(&mut self.doc, platform, "tasks")?
        } else {
            self.doc["tasks"]
                .or_insert(Item::Table(Table::new()))
                .as_table_mut()
                .ok_or_else(|| {
                    miette::miette!("target table in {} is malformed", consts::PROJECT_MANIFEST)
                })?
        };
        let depends_on = task.depends_on();

        for depends in depends_on {
            if !self.manifest.tasks.contains_key(depends) {
                miette::bail!(
                    "task '{}' for the depends on for '{}' does not exist",
                    depends,
                    name.as_ref(),
                );
            }
        }

        // Add the task to the table
        table.insert(name.as_ref(), task_as_toml(task.clone()));

        self.manifest.tasks.insert(name.as_ref().to_string(), task);

        self.save()?;

        Ok(())
    }

[ruben-arts]

Closed with #572