Metadata keys containing numeric characters break metadata fontification

[saf-dmitry]

Expected Behavior

YAML/TOML/MultiMarkdown metadata keys containing numeric characters, like this

---
key 1: value
key 2: value
---

should not break proper detection and fontification of metadata block. Numeric characters are allowed in metadata keys according to specifications of the all above-mentioned formats.

Actual Behavior

YAML/TOML/MultiMarkdown metadata keys containing numeric characters break proper detection and fontification of metadata block.

Steps to Reproduce

1. Add folowing YAML metadata to the top of a file:

   ---
   key 1: value
   key 2: value
   ---
   

2. See the metadata block not detected.

Backtrace

Software Versions

• Markdown Mode: From Git on 2022-07-23
• Emacs: v26.3
• OS: macOS v10.14.6