Fix prose citations using "&" instead of "and" between author names

[minco3]

• Prose citations (form: "prose") currently use "&" to join author names, but should use "and". This affects any CSL style with and="symbol" (e.g. APA).
• Adds a use_text_and flag to WritingContext, set during prose author rendering, which causes the name joiner to use the localized "and" term instead of "&"
• Updates the existing issue_243 test to expect "and"

The approach works and matches the existing pattern of toggling context flags in WritingContext, though it's admittedly a bit of a bolted-on solution. Open to suggestions on a cleaner approach.

Closes #454