fix(build): style insert order for UMD builds (fix #13668)

[binary-koan]

Description

Fixes #13668

The previous fix in #13266 can break the insertion order for UMD builds when a dependency containing the string execute: is added. This updates the logic to only check for execute: when System.register is found, which should be a more robust way to find the insertion point.

TLDR

• Legacy builds which generate System.register calls will insert the CSS in the execute function
• Builds which don't have that format will consistently insert the CSS at the top of the IIFE

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.
• Read the Pull Request Guidelines and follow the PR Title Convention.
• 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).
• Ideally, include relevant tests that fail without this PR but pass with it.

---

Submitted with StackBlitz Codeflow.

[stackblitz]

Run & review this pull request in StackBlitz Codeflow.

[rishabhgrg]

Hey @sapphi-red , just a gentle bump here :) We are blocked on using SWR till this is fixed as this bug causes all injected CSS styles to disappear as soon as SWR is used.

[patak-dev]

@binary-koan you don't need to be blocked by this PR. Use https://pnpm.io/cli/patch or https://www.npmjs.com/package/patch-package so you can use this PR in your project until it is released.

[binary-koan]

@sapphi-red Makes sense! Updated to do the right thing for both (inserting after "use strict" for UMD and after "execute:" for legacy) - seems to work nicely in the reproduction cases for both the previous issue and my issue

[sapphi-red]

Thank you!