Add support for getting previous extended grapheme cluster break

[ShraddheyaS]

Resolves #362.

Checklist

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• Read, understood, and followed the contributing guidelines, including the relevant style guides.
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Description

What is the purpose of this pull request?

This pull request:

• Adds support for getting the previous extended grapheme cluster break in a string before a specified position.

Related Issues

Does this pull request have any related issues?

This pull request:

• resolves RFC: Add support for getting previous extended grapheme cluster break #362
• fixes Algorithm to remove last string character is not Unicode aware #297
• fixes Algorithm to reverse a string should account for extended grapheme clusters #299

Questions

Any questions for reviewers of this pull request?

No.

Other

Any other information relevant to this pull request? This may include screenshots, references, and/or implementation notes.

Just as next-grapheme-cluster-break imagines a break at the end of the string (return value str.length), this package imagines a break at the start of the string (return value -1).

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@stdlib-js/reviewers

[kgryte]

This PR has similar concerns as #361 . Based on the resolution of those concerns, this PR may need to be similarly updated.

[ShraddheyaS]

Closing as plan to include this auxiliary functionality in the next-grapheme-cluster-break package.

[kgryte]

Sounds good!