Allow scoped snippet

[octref]

Sublime allows snippet for particular scopes: http://docs.sublimetext.info/en/latest/extensibility/snippets.html

This is particularly useful for embedded languages like vue.js:

<template>
  <div id="app">
    Clicked: {{ $store.state.count }} times, count is {{ evenOrOdd }}.
    <button @click="increment">+</button>
    <button @click="decrement">-</button>
    <button @click="incrementIfOdd">Increment if odd</button>
    <button @click="incrementAsync">Increment async</button>
  </div>
</template>

<script>
import { mapGetters, mapActions } from 'vuex'
export default {
  computed: mapGetters([
    'evenOrOdd'
  ]),
  methods: mapActions([
    'increment',
    'decrement',
    'incrementIfOdd',
    'incrementAsync'
  ])
}
</script>

[alexdima]

This can be implemented from an extension that registers a completion item provider that is smart enough to suggest certain snippets only in certain locations.

[siegebell]

@alexandrudima I see no generic way for an extension to do this until #580 is resolved.

[octref]

@alexandrudima Well, now the user who wants to make a snippet only available to a certain part of his code has to:

• Re-parse the code using vscode-textmate on each document change (which can be slow)
• Get a parser for the snippets (the one bundled with VS Code can't be easily used standalone)
• Write a CompletionItemProvider that provides the parsed snippets as CompletionItem if scope matches.

That's a lot of work and the extension will be slow.

[alexdima]

@octref It is not in our best interest to further leak TM scopes in APIs or build more and more features on top of TM scopes:

We want at one point to be able to colorize using something else that doesn't have the problems and baggage of TM. For example, TM coloring is one our biggest perf detractors.

[octref]

OK I'll close this as out-of-scope since I don't see we replacing TM anytime soon.
Sublime does have this since they have their own rule engine.
Meanwhile I'm doing scoped snippets in Vue using my own parser.