Improved WSL 2 detection and browser resolution

[yhatt]

Improved WSL2 detection by using multiple heuristics.

Marp CLI will find out the browser for PDF/PPTX/image conversion automatically from the host OS (Windows). It works in WSL 1 but Puppeteer in WSL 2 cannot communicate with the browser spawned in the host OS.

I've updated isWSL() utility method for more reliable WSL 2 detection. We have no longer resolved Puppeteer from the host OS if WSL 2 was detected.

WSL2 detection is based on multiple heuristics on WSL environment:

• If there are both of WSL_DISTRO_NAME and WSL_INTEROP environment values.
• Else if the string of /proc/version is including Microsoft-Standard-WSL2.
• Else if it shows that the kernel was compiled by GCC v8 and later versions.

Related to marp-team/marp#113, and may resolve marp-team/marp#247.