Add language: Reach

## Language name
Reach

## URL of example repository
You can see a lot of `.rsh` files in places like https://github.com/reach-sh/reach-lang/tree/master/examples.

https://github.com/reach-sh/reach-lang/blob/master/examples/api-call/index.rsh is one example `.rsh` file.

## URL of syntax highlighting grammar
This language is very similar to JavaScript. Reach doesn't support `jsx` syntax. It also doesn't have support for `let` as a keyword. It is otherwise basically exactly the same as [`extensions/javascript/syntaxes/JavaScript.tmLanguage.json`](https://github.com/microsoft/vscode/blob/main/extensions/javascript/syntaxes/JavaScript.tmLanguage.json).

## Most popular extensions
[Reach IDE](https://marketplace.visualstudio.com/items?itemName=reachsh.reach-ide), which is actually what I work on

## Detected language
RenderScript