feat: allow to enhance only once

[kptdobe]

If sampleRUM.enhance() is called twice, enhancer code will be injected twice and we might have duplicate events.

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Commits

* [3074db6](https://github.com/adobe/helix-rum-js/commit/3074db6c0d62283086e0f73f9d991afdc5804225) :white_check_mark: (latest)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (c67a739) to head (3074db6). Report is 3 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #220 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 2 2 Lines 179 183 +4 ========================================= + Hits 179 183 +4 ```

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[adobe-bot]

:tada: This PR is included in version 2.7.0 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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