Added $startIndex param to Pos method

[nedmas]

Description

I've added $startIndex param to the Pos method with a default value of 1.

Manual testing steps

Loop through elements in an elemental area and print the results of $Pos(1).

Issues

• Support $startIndex param on Pos method #1221

Pull request checklist

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• This change is covered with tests (or tests aren't necessary for this change)
• Any relevant User Help/Developer documentation is updated; for impactful changes, information is added to the changelog for the intended release
• CI is green

[GuySartorelli]

Thanks for submitting this PR.

You ticked the "The commit messages follow our commit message guidelines" and "This change is covered with tests (or tests aren't necessary for this change)" checkboxes, but the commit message is wrong and there are no tests.

Please add the API prefix to the commit message and add unit tests.

Please also check if there's any documentation in this repo that needs to be updated, and submit a pull request to the 6 branch of https://github.com/silverstripe/developer-docs to update the CMS 6 changelog.

[GuySartorelli]

Suggested change

	public function Pos($startIndex = 1)
	public function Pos(int $startIndex = 1)

[GuySartorelli]

@nedmas Are you still interested in having this feature added to CMS 6? If so, please make the requested changes.
If the changes aren't made, the PR may be closed.