Added support for word "mail merge" fields

[cjbaar]

Recently used this library for a project which required inline text substitution. Legacy document was already built using pre-defined merge fields, and given it's large size, we did not want to reformat to take advantage of the existing setValue() method. I created some new functions to allow replacement of text using the pre-defined merge fields, allowing the same document to be converted using both the older manual method and the new programmatic (PHPWord) method. This is based on a limited set of Word merge documents, but so far all replacements are successful.

[gfobe]

The RegEx searching for w:instr=" MERGEFIELD does not trigger in my document Placeholder.docx

[cjbaar]

@gfobe: Your document is using a merge field format that we did not encounter on our project. It looks like there are two different types of merge field in Word: simple and complex. We currently only have used the complex method. According to what I have read, if there is only one merge field on a document, it default to simple. As a short work-around, try adding a second merge field in the document and see if that changes things. (You should see fldChar tags instead of fldSimple)

[FBnil]

See also #1050

[github-actions]

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