Fix serving index files

[n-ari]

Fixed URLs serving index files (index.md, PITCHME.md) in server mode.

Problem

Write presentation files in directories like below:

• serverRoot/files/index.md : markdown file for Marp
• serverRoot/files/image.png : image file, used in index.md as ![](image.png)

And run marp-cli as server mode like marp --server serverRoot/.

Then http://localhost:8080/ shows files and directories in serverRoot by serve-index.
In the above settings, a link on that page is http://localhost:8080/files.

When access to that, serverRoot/files/index.md is converted and shown in a browser, but serverRoot/files/image.png is not shown in the presentation because the presentation address is /files and ![](image.png) is linked to /image.png, not /files/image.png.

When access to http://localhost:8080/files/ (with a trailing slash), serverRoot/files/image.png is shown.

For compatibility with other modes, ![](image.png) should be linked to /files/image.png, not /image.png I think.

Solution

To fix this, there are 2 solutions:

1. Add a trailing slash to the links on the serve-index page.
2. Redirect to URL with a trailing slash.

In this PR, fixed the template file src/server/index.pug to add a trailing slash to the directory links.

[yhatt]

Yeah, it's reasonable. Thank you 👍