Support non-rstudio viewer

[renkun-ken]

Since vscode-R enables VSCode to show htmlwidgets in a WebView via REditorSupport/vscode-R#150, it should work with most htmlwidgets. However, as reported in REditorSupport/vscode-R#158, kableExtra does not use getOption("viewer") but directly use rstudioapi::isAvailable() and rstudioapi::viewer, which imposes dependency on RStudio viewer.

A better approach is to use getOption("viewer"). In RStudio, it is set to use RStudio Viewer, and in VSCode, it is set to use VSCode WebView, in other cases it is NULL.

[renkun-ken]

Just saw latest print.kableExtra uses htmltools::html_print which uses getOption("viewer"), and it works with VSCode WebView now.