Drop support for VS Code < 1.36

[yhatt]

We are considering to drop support for VS Code 1.31-1.35, that are using Electron which is required polyfill to render Marp preview.

We must leave ugly logics to apply polyfill for the correct preview in old versions. (e.g. #8, #19) In addition, The least version 1.31 cannot use auto fix command in Diagnostic API.

marp-vscode/src/diagnostics/deprecated-dollar-prefix.ts

Line 112 in f9605fe

// act.isPreferred = true // TODO: Require VS Code >= v1.32

Thus, it's hard to maintain old compatibility while using features for the recent version. VS Code has auto-updation into the latest version, and we can't think that there are stuck users in the previous version.