Allow Views table caption field to support tokens from the first row of results

Here's a first attempt at this. Adds tokenize process, overrides the usesTokens method from the parent plugin and changes views.theme.inc to use the newly created getCaption() method.

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Thanks for opening

New return functions should be typehinted
Will need test coverage

Also new functions will need a change record.

Thanks!

Updated the MR with relevant functions doc comments. Retaining 'needs work' for tests and CR.

Thanks!

Added some comments. Other field allow this in these settings, so feels ok to allow this for the caption too.

As pointed out in #6, this needs test coverage

This should target the 'main' branch (called 11.x for now)