Support curly braces for code fence language specifier in Markdown

[arwedus]

Description

To render plantuml in Markdown Preview Enhanced, I need to write the following syntax:

```plantuml
@startuml

Please support the language specifier Syntax with curly braces as well: {plantuml} Or even better: Make this a config option.

Use Case:

Language specifier with curly braces are parsed as a directive in Sphinx-doc using the MyST Markdown parser. The directive is then parsed by sphinxcontrib.plantuml, which renders the diagram. Language specifier without curly braces just generates a code block with marked-up text, as per Markdown specification.

I therefore would use the language specifier syntax with curly braces, which MyST translates into a reStructuredText directive.

Example:

```{plantuml}
@startuml

ℹ️ Actually, sphinxcontrib.plantuml expects the directive name "uml", but it's easy to add an alias "plantuml" for the directive.

[Geocali]

@arwedus did you find a workaround for this?

[arwedus]

@Geocali : I don't think there is a workaround for this.

[Geocali]

In src/markdown-it-plantuml/rule.ts

import * as markdowIt from 'markdown-it';

export function plantumlWorker(state: any) {
    // debugInfo(state.tokens);
    let blockTokens: markdowIt.Token[] = state.tokens;
    for (let blockToken of blockTokens) {
        if (blockToken.type == "fence" &&( blockToken.info.startsWith("plantuml")|| blockToken.info.startsWith("puml"))) {
            blockToken.type = "plantuml";
            // always render as <img> for maximum compatibility:
            // https://github.com/qjebbs/vscode-markdown-extended/issues/67#issuecomment-554996262
            blockToken.tag = "img";
            // if (state.env && state.env.htmlExporter) { // work with markdown extended export, solve #253
            //     blockToken.tag = "object";
            // } else {
            //     blockToken.tag = "img";
            // }
        }
    }
}

We could probably replace

(blockToken.type == "fence" &&( blockToken.info.startsWith("plantuml")|| blockToken.info.startsWith("puml")))

by

(blockToken.type == "fence" &&( blockToken.info.startsWith("plantuml")|| blockToken.info.startsWith("puml"))) || blockToken.info.startsWith("{uml}")||