Some dates in front matter (yaml) are not being marked as timestamps

[alexr00]

From @Tyriar in microsoft/vscode#84435

---
a: b
a: 2019-10-1
a: 2019-10-10
---

Version: 1.41.0-insider (user setup)
Commit: 27cd33d891eb23e370b672e751be9fe8b3748ac1
Date: 2019-11-08T19:12:35.358Z
Electron: 6.1.4
Chrome: 76.0.3809.146
Node.js: 12.4.0
V8: 7.6.303.31-electron.0
OS: Windows_NT x64 10.0.18362

[RedCMD]

That would be a design decision with the ISO 8601 standard https://www.w3.org/TR/NOTE-datetime that YAML:1.0 and YAML 1.1 used https://yaml.org/type/timestamp.html
YAML 1.2 no longer (inherently) supports timestamps