feat(Ohkami, Mount): add `ServeDir`, `ServeFile` struct & extend `Route::By` and remove `Route::Mount`

[kanarus]

• Introduce explicit ServeDir and ServeFile struct:
  • each exposes new(path) -> Self method and builder-style options methods.
  • each implements Into<Ohkami> that convert it into an Ohkami instance that serves a directory or a file.
• Extend Route::By to take impl Into<Ohkami>, instead of Ohkami.
• Then Route::Mount special method is not needed at all. Remove it to simplify Route interface.

before:

use ohkami::{Ohkami, Route};

async fn main() {
    Ohkami::new((
        "/api".GET(async || "Hello, Ohkami!"),
        "/".Mount("./public")
            .serve_dotfiles(true)
            .omit_extensions(["html"])
    )).run("localhost:3000").await
}

after:

use ohkami::{Ohkami, Route};

async fn main() {
    Ohkami::new((
        "/api".GET(async || "Hello, Ohkami!"),
        "/".By(
            ohkami::ServeDir::new("./public")
            .serve_dotfiles(true)
            .omit_extensions(["html"])
        )
    )).run("localhost:3000").await
}

[kanarus]

• related to fix(Mount): handle buffering correctly #234