Create `theme_inline_list()`

[docwilmot]

Description of the need

We have theme functions that help create lists, such as theme_item_list() but comma-separated lists need to be created with custom code.

Proposed solution

Create theme_inline_list() to allow easier DX for comma or other character-separated lists.

Alternatives that have been considered

Continue to do this manually. PHP's explode() makes this easy but a custom theme function would allow wrapper HTML, and theme overrides.

Draft of feature description for Press Release (1 paragraph at most)

Backdrop now includes theme_inline_list() to create lists of strings separated by chosen string characters, such as commas.