Heron's Formula for Area of any (valid) triangle.
## Description
Adding heronArea.js and adding the corresponding entry in tag_database. More info on Heron's formula (and why it works) here: https://en.wikipedia.org/wiki/Heron%27s_formula. This is exceptionally helpful for simplifying any math being done inside your JS, and abstracts away any altitude calculations being done before the (1/2)bh calculation often used instead.
## PR Type
- [x] Snippets, Tests & Tags (new snippets, updated snippets, re-tagging of snippets, added/updated tests)
- [ ] Scripts & Website & Meta (anything related to files in the [scripts folder](https://github.com/30-seconds/30-seconds-of-code/tree/master/scripts), how the repository's automated procedures work and the website)
- [ ] Glossary & Secondary Features (anything related to the glossary, such as new or updated terms or other secondary features)
- [ ] General, Typos, Misc. & Meta (everything else, typos, general stuff and meta files in the repository - e.g. the issue template)
## Guidelines
- [x] I have read the guidelines in the [CONTRIBUTING](https://github.com/30-seconds/30-seconds-of-code/blob/master/CONTRIBUTING.md) document.
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## Description
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Adding a snippet to fetch all images from within a desired element.
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## PR Type
- [x] Snippets, Tests & Tags (new snippets, updated snippets, re-tagging of snippets, added/updated tests)
- [ ] Scripts & Website & Meta (anything related to files in the [scripts folder](https://github.com/30-seconds/30-seconds-of-code/tree/master/scripts), how the repository's automated procedures work and the website)
- [ ] Glossary & Secondary Features (anything related to the glossary, such as new or updated terms or other secondary features)
- [ ] General, Typos, Misc. & Meta (everything else, typos, general stuff and meta files in the repository - e.g. the issue template)
## Guidelines
- [x] I have read the guidelines in the [CONTRIBUTING](https://github.com/30-seconds/30-seconds-of-code/blob/master/CONTRIBUTING.md) document.
changed 'Returns all indices of val in an array' to 'When val is not found, return an empty array' for test that searches for val in an array without val
