feat: add cross-reference

[iamgio]

This PR introduces cross-references:

• Headings

  # Text {#my-ref}

• Figures

  ![alt](img.png "Caption") {#my-ref}

• Tables

  | A | B | C |
  |---|---|---|
  | D | E | F |
  "Caption" {#my-ref}

• Code blocks

  ```javascript {#my-ref}
  console.log("Hello");
  ```

• Custom .numbered blocks:

  .numbered key:{abc} ref:{my-ref}
      Hello
  

Support for math equations (#185) will be added in a subsequent PR, which will also introduce equation numbering.

Elements can be cross-referenced via the .ref function. Cross-referencing works best when combined with numbering.

.numbering
  - headings: 1.1

# Title

## First

For more info, see .ref {second}.

## Second {#second}

...

Output:

...
For more info, see Section 1.2.
...