Update HighLevelGoals.md to reflect Wasm's current goals

Wasm has evolved beyond asm.js, the inital MVP, and the first few feature proposals. Update the HighLevelGoals.md document to reflect how these goals have been generalized in practice to supporting more languages and more features. There is discussion in https://github.com/WebAssembly/design/issues/1349 about whether we should preserve the historic design documents. That may be good to do, however I believe doing it well would require some manual curating. Historically the documents weren't always kept in sync with each other, and we'd need to figure out how to present the information. The git history remains available, in case anyone would like to put together a PR to curate it.