Convert to custom Suggestion type & filter text with filterText key

[Aerijo]

Converts the internal uses of ac.AnySuggestion to the new Suggestion, which is an extended version of the original. In particular, it has the property filterText, used for custom filtering. It also makes it easier to add features, such as the replacement prefix tracking in #254

Regarding filtering using filterText, it seems not even VS Code had a solid idea on the difference between filterText and sortText for a time

• Inline snippets should have lower precedence then actual properties if no match microsoft/vscode#26944 (comment)
• Vscode 1.12 no longer obeys 'sortText' in completion items (from language server) microsoft/vscode#26096 (comment)

The latest I could find is in that last issue, where they say [paraphrasing] "use sort text for no prefix" and "use filterText for when there's a prefix" (and they base on other things like match index). The source code also supports using filterText to sort as well when there's a prefix.

[Aerijo]

@50Wliu The package-lock.json looks quite different to the one you committed earlier, specifically the version range symbols have been removed. Is this fine?

[50Wliu]

Yeah, that's fine. Just commit whatever the result of npm install is.