Preview Enhanced：right click menu enhanced

[kestiny]

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I'm using asciidoc in the vscode, it is a very powerfull productivity tool。Before this，I'm using markdown in the vscode, i think there are some feature vary userfull when use the preview function。Therefore it is hoped to increase asciidoctor-vscode preview.

• What you're trying to do add right click menu when preview asciidoc in the vscode, include features:

1. open in browser
2. export(html5, pdf, docbook...)
3. preview theme

• How you think this extension could handle this

1. open in browser
   • save the preview asciidoc to html file with /tmp path
   • open the /tmp html file in default browser
2. export map the current command(Export as PDF,Save as HTM,Save to Docbook) to the right click menu
3. preview theme
   • light
   • dark
   • follow the system

• Possible as shown in the following: