Adds a new script extension that allows tracking interactions with specific HTML elements on a website. For example - to track link clicks on one specific `<a>` element, you can tag it like this:
```html
<a href=... class="plausible-event-name=<your_event_name>">
```
And you can also tag the link with custom property names and values:
```html
<a href=... class="plausible-event-name=<your_event_name> plausible-event-<your_custom_prop>=<your_value>">
```
Tagging a link as above will send a custom event with the given name and props, if a `click` or `auxclick` browser event happens, and targets the link element.
The tracking behavior is somewhat different based on the HTML element type:
- `<a>`
- triggers on `click` and `auxclick` events
- intercepts navigation based on the same rules as `outbound-links` and `file-downloads`
- `<form>`
- triggers on `submit` event
- always intercepts navigation (calls `form.submit()` after preventing default and sending the Plausible event)
- other (`<img>`, `<button>`, `<span>`, `<div>`, `<h2>`, etc ...)
- triggers on `click` and `auxclick` events
- does not prevent default to intercept possible navigation. Simply calls Plausible with the event name and props read from the element class list.
* moved custom event code to the bottom + fix indentation
* add handlebars helper fn + extract getLinkEl fn
* extract isOutboundLink function
* extract shouldFollowLink function
* remove middle and click variables
* use only one click handler for outbounds and downloads
* extract sendLinkClickEvent function
* add error handling when script compilation fails
* use callback instead of fixed timeout
* do not prevent default if externally prevented + test
* add more tests
* generate tracker files in priv/tracker/js
* update changelog
* requested changes after review
* regenerate tracker files
* use return instead of else if
* move middleMouseButton outside the function
