API for creating null-terminated string data variables

Sometimes analysis misses strings, or you're dealing with a format that you know has strings at certain offsets, etc.

There is no API for "create a (null-terminated) string data variable starting at this address". As such, we've reinvented this behavior in a few places in the core (linearview.cpp:5845, tokenizedtextview:1877, and more). So this is really a two part feature request:

1. Consolidate any areas where we do this into the core into one true implementation.
2. Make an API available so plugins, etc. don't have to re-implement the whole "keep reading until you find a null terminator then construct the appropriate array type" process.

This API should support UTF-{16,32} strings as well, which should be easy via an optional parameter.

Start by comparing the existing string-variable logic at linearview.cpp:5845 and tokenizedtextview:1877, then locate other core implementations. Consolidate the behavior into one core implementation and expose an API for plugins, including null-terminated UTF-16 and UTF-32 strings. Done means these call sites use the shared implementation and the public API supports the requested encodings.