fix(html): handle offset magic-string slice error

[bluwy]

Description

fix #15327

magic-string .slice() may throw an error if the sliced string has removed content. I added a try-catch to fix it.

I was thinking if we could check against the plain string instead, but seems risky if there's added content later in magic-string.

I also refactored the code a bit.

Additional context

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines, especially the Pull Request Guidelines.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Update the corresponding documentation if needed.
• Ideally, include relevant tests that fail without this PR but pass with it.

[stackblitz]

Run & review this pull request in StackBlitz Codeflow.