epub: fix send-to-kindle failure on notes.html

[chen-charles]

Kindle Previewer indicates the following error,

"Type","Description"
"Error","E1005: Could not access file. in file: <path>\OEBPS\notes.html","","","",""
"Status","Book conversion failed."

Calibre editor points to the same issue,

This PR,

1. excludes notes.html reference if data.notes is not valid
2. fixes compile on case-sensitive fs (e.g., ubuntu)
3. fixes prettier having trouble with \\ on unix fs
4. pins @types/har-format to 1.2.8: typescript 3.9.10 does not compile with newer har-format

[xuejianxianzun]

Thanks! I've submitted a new version with this update and it's under review now.