GFM strikethrough syntax

[yhatt]

The old Marp app is supported GFM stirikethough ~~like this~~, but Marp Next is not. We are listening the quiet feedback from some users want us to support strikethrough syntax.

Marp Next's policy is "Avoid breaking CommonMark as much as possible" and strikethrough syntax is against our policy. But if the new syntax is useful and well-known feature in Markdown community widely, Marp Core allows additional feature like breaking CommonMark syntax. (e.g. Math typesetting)

So I think strikethrough syntax ~~ might as well add to Marp Core.

It is good for first contribute because it has only to turn on strikethrough rule in markdown.enable().

marp-core/src/marp.ts

Line 63 in f1a59d8

this.markdown.enable(['table', 'linkify'])