Allow Formatting of Read Time String

## Summary

When inserting a formatted read-time string, it would be useful to be able to format the default string format.

## Feature Request

Add a setting that would expect a string with a `time` placeholder that would format inserted read-time strings. In the `settings.json` file, you could set something like this:

```
"readTime.defaultString": "Article read time: ${time}"
```

Then when the string is inserted, the result would be:

`Article read time: 20 minutes`

Related to #13

Start by locating where the extension reads settings and inserts the read-time string; the issue identifies settings.json and the read-time insertion path but no source files or tests. Trace the existing default string, then verify that a configured ${time} placeholder produces the requested formatted output.

Written by the indexing model from the issue text.