Add completion providers

[Daniel-Knights]

What

Add support for:

• Attribute completion within opening tags
• CSS property/value completion within mj-style tags
• HTML tag completion within mj-text tags

How

Create a new Completion class that utilises VSCode's registerCompletionItemProvider and add a folder with the necessary snippets.

Examples

Attribute completion within opening tags

CSS property/value completion within mj-style tags

HTML tag completion within mj-text tags

[iRyusa]

Looks good to me @kmcb777 can we release that ?
We'll see if we can improve it by using component definition later

[dnuanes]

Hey Daniel, thanks for the PR. This is a fantastic feature.

[Daniel-Knights]

Hey, thanks @dnuanes