Syntax Highlighting for ICU message templates

[CrazyChris75]

The Unicode ICU message templates (see https://unicode-org.github.io/icu/userguide/format_parse/messages) are used in browsers and libraries (for example format.js and react-intl, see https://formatjs.io/docs/intl-messageformat)

Syntax hightlighting for placeholders and parameters in these kinds of templates would be great:

Maybe there is already a supported language that would include such single curly bracket placeholders
like {var}
or {price, number, ::currency/EUR}

If not, ICU messages being a standardized templating system, it would be great to include it in supported languages of prism.js and I think it should be based on "markup"

[bertho-zero]

#2745

[RunDevelopment]

I think, I forgot to close this after #2745. Please comment if this need to be reopened.