`if` statement in comment not shown as comment (syntax highlighting)

**Describe the bug**
A clear and concise description of what the bug is.

**To Reproduce**
Steps to reproduce the behavior:
1. Create a new workflow file and paste in the below code.
2. Observe how `if: ...` is highlighted incorrectly.

```yml
name: Example Workflow File
on:
workflow_dispatch:
jobs:
test:
runs-on: ubuntu-latest
steps:
- name: A fake step
run: echo Hello World
# This is a block comment
# The if: with a colon after it is not highlighted properly
# An if without a colon is fine.

```

**Expected behavior**
A clear and concise description of what you expected to happen.
The `if:` and following text should remain grey like a comment.

**Screenshots**
If applicable, add screenshots to help explain your problem.


**Extension Version**
`v0.26.2`

**Additional context**
Add any other context about the problem here.

This seems slightly related to #158

まず、issue の YAML 例を使い、extension v0.26.2 で新しい workflow ファイルにハイライトの問題を再現します。`if:` を含むコメントと、`if` にコロンがないコメントを比較します。`if:` のテキストとそれに続くコメントのテキストが、workflow の他のハイライトを変更せずに灰色のままになれば完了です。

索引モデルが issue の本文から書いたものです。