Add hyperlinks to Theme format elements

[foster999]

Is your feature request related to a problem? Please describe.
I'd like to be able to set the formatting of hyperlinks and annotation references in the Theme. E.g. blue, underlined hyperlinks and superscript annotations.

Describe the solution you'd like
As other Theme elements

Will need to convert string to rich text for annotation references. Hyperlinks might use regex for starting with url or HYPERLINK furmula.

Related to #135

[rowanhemsi]

As of v1.0.0, hyperlinks are formatted as underlined blue text. In line with the Analysis Function guidance published 2021-06-30, superscript annotations are not accessible, but future customisation of the hyper link formatting could be supported, subject to the following constraints:

Link text should be underlined by default. If it is not, the accessibility guidelines say you must consider colour contrast with surrounding text and a “visual cue” that appears on mouse hover and keyboard focus.