feat: add optional merge method to decoration object

[yf-yang]

Description
Currently, when decorations overlap with each other and they are merging with each other, Object.assign is used.
However, if two decorations with the same key has different values, one of the decoration will overwrite the other.

// text content: `them are happy`
// decoration 1:
{
  anchor: {
    path: [0],
    offset: 0,
  },
  focus: {
    path: [0],
    offset: 1,
  },
  message: '`t` should be upper case.'
}

// decoration 2:
{
  anchor: {
    path: [0],
    offset: 0,
  },
  focus: {
    path: [0],
    offset: 4,
  },
  message: 'The correct word is `they`.'
}

In this example, two messages cannot be shown together due to Object.assign overwriting behavior.

Example
A GIF or video showing the old and new behaviors after this pull request is merged. Or a code sample showing the usage of a new API. (If you don't include this, your pull request will not be reviewed as quickly, because it's much too hard to figure out exactly what is going wrong, and it makes maintenance much harder.)

After adding a function merge to the decoration, two messages can be merged together:

// text content: `them are happy`

const merge = (leaf, decoration) => {
  const { messages, ...rest } = decoration;
  leaf.messages = [...(leaf.messages ?? []), ...messages];
  Object.assign(leaf, rest);
}

// decoration 1:
{
  anchor: {
    path: [0],
    offset: 0,
  },
  focus: {
    path: [0],
    offset: 1,
  },
  merge,
  messages: ['`t` should be upper case.']
}

// decoration 2:
{
  anchor: {
    path: [0],
    offset: 0,
  },
  focus: {
    path: [0],
    offset: 4,
  },
  merge,
  messages: ['The correct word is `they`.']
}

Checks

• The new code matches the existing patterns and styles.
• The tests pass with yarn test.
• The linter passes with yarn lint. (Fix errors with yarn fix.)
• The relevant examples still work. (Run examples with yarn start.)
• You've added a changeset if changing functionality. (Add one with yarn changeset add.)

[changeset-bot]

🦋 Changeset detected

Latest commit: b3e18e5

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

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[dylans]

Nice improvement, thank you!