Trailing slash after the HTML file name

[whitphx]

For example, when the mountable code is used in http://localhost:3000/test.html, the URL will be overwritten to http://localhost:3000/test.html/ on which the trailing slash / is added.
This behavior is originally necessary for page transitions on MPA (ref: #176, https://github.com/streamlit/streamlit/blob/1.12.2/frontend/src/App.tsx#L656-L665 ), but it became a problem in the case of stlite, which can be accessed through URLs endings with *.html.
So, a special care for such cases is needed.

[whitphx]

https://github.com/whitphx/streamlit/blob/d1d2b1623faded933639cc91898c17b9d1c7be46/frontend/src/App.tsx#L667-L669

This pageUrl should be fixed.

• If it has a trailing slash like "test.html/", remove it to be "test.html".
• However, If it is "/", keep it, because an empty string "" does not affect when passed to history.pushState()