Have a better parseComponent

[octref]

Have a typed version of vue-template-compiler.parseComponent.
We still need the html parser, but the top level region dividing can be done using a simpler version.

Maybe base it on parse5. Same approach as https://github.com/prograhammer/vscode-tslint-vue.
This can be reused by a lot of third parties, such as ESLint, TSLint, formatters, etc which are only interested in a specific region of .vue file.
Should also output offsets, so linting error can be shown at right place, formatted code can be placed to right place, etc.

Ideally later we'd also want to use Vue's own parser for <template> region, so we get a richer AST for vue specific stuff.

In short term, we should disable scaffoldCompletions in custom blocks.

[HerringtonDarkholme]

I actually think our version of parseComponent is better. Note vue-template-compiler isn't designed for bad input, say, users are still typing their template code.

Vue's parser is intentionally designed for browser. That means a lot of AST info isn't available, in exchange for smaller runtime code.

[octref]

This should fix #430 too.