We should support string indexes for all marker string types

To save space in the profile, we can deduplicate strings in marker fields by using the `unique-string` field type. However, how do I use deduplicated URLs? Or deduplicated sanitized strings? I have to pick either `url` or `unique-string`, there's no `unique-url` or `unique-sanitized-string`:

https://github.com/firefox-devtools/profiler/blob/af62509321b9610b7acacb7e4094eccd99945a35/src/types/markers.js#L20-L37

I think it would be better to remove the `unique-string` type and turn all string fields into string indexes.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-800)

Start with src/types/markers.js lines 20-37, which lists the marker string field types. Trace how these types are consumed to identify all affected marker fields; done means deduplicated URLs and sanitized strings can use string indexes without retaining a separate unique-string type.

Written by the indexing model from the issue text.