fix: readFile on windows (works cross-platform)

[joshuaslate]

Using path instead of filepath fixes an issue on Windows where the template files could not be found. I tested these changes on macOS and Windows and they worked for me.

Relevant docs:

https://pkg.go.dev/embed

...
The path separator is a forward slash, even on Windows systems.
...

https://pkg.go.dev/path/filepath

The filepath package uses either forward slashes or backslashes, depending on the operating system. To process paths such as URLs that always use forward slashes regardless of the operating system, see the [path](https://pkg.go.dev/path) package.