Centralize all file system access in VLS

[octref]

Motivation

Currently Vetur's file system access is scattered throughout the code. However, we also use TextDocuments from https://github.com/Microsoft/vscode-languageserver-node to track LSP documents, and with the new FileEvents generated through VS Code file watcher, it's time to put these all together.

Benefits

• All file access / events in one place. Easier to track, log, do perf analysis, etc
• Possible to use in-memory / virtual FS system with VLS
• Easier to manage the "virtual files" we generate for each region in Vue files
• Possible to reduce chattiness of LSP by switching to Incremental Syncing
• Possible to listen to changes by granularity of Vue regions (as compared to currently letting each file change triggering all features in file)

Fixes

• Vue file. Intellisense shows outdated list of exported members #355
• Have a better parseComponent #532

Unblocks

• Multi root support #424

[s-KaiNet]

Looking forward to see this issue fixed with PR and new version published.
Thank you for your great efforts you invest in this (and other) OSS projects!

[octref]

Outdated and won't do any more.