add support for custom templates

[xdavidson]

This MR allows user-defined custom templates to be provided at conversion time.
To use a pug template:

• On a terminal, $ npm i pug
• In Settings -> AsciiDoc -> Open Preview, define path to pub templates

[ggrossetie]

@xdavidson I left a few nitpicks but otherwise it looks very good, thank you!

[ggrossetie]

As a follow-up we should hide this option when running in Visual Studio Code for the Web but we can do that later.

[ggrossetie]

Linter is unhappy:

/home/runner/work/asciidoctor-vscode/asciidoctor-vscode/src/asciidocParser.ts:171:15: ["template_dirs"] is better written in dot notation. [Error/dot-notation]
/home/runner/work/asciidoctor-vscode/asciidoctor-vscode/src/asciidocParser.ts:171:15: Strings must use singlequote. [Error/quotes]
/home/runner/work/asciidoctor-vscode/asciidoctor-vscode/src/asciidocParser.ts:273:[11](https://github.com/asciidoctor/asciidoctor-vscode/runs/7753730673?check_suite_focus=true#step:5:12)2: A space is required after ','. [Error/comma-spacing]
/home/runner/work/asciidoctor-vscode/asciidoctor-vscode/src/asciidoctorWebViewConverter.ts:121:31: Missing trailing comma. [Error/comma-dangle]

You can run npm run lint to check locally. You can also use npm run lint -- --fix to try to automatically fix lint issues.

[xdavidson]

You can run npm run lint to check locally. You can also use npm run lint -- --fix to try to automatically fix lint issues.

Thanks for the pointers. It's fixed.

[xdavidson]

As a follow-up we should hide this option when running in Visual Studio Code for the Web but we can do that later.

Good point!