This time, functions for decoding `wstr` into various types and the
`ToCString` trait are extracted.

Part of the wider goal of slimming down the main library to improve
incremental build performance and reduce dependency cycles.

Previously, we chose the ellipsis character/string based on the locale.
We now assume a UTF-8 locale, and accordingly always use the Unicode
HORIZONTAL ELLIPSIS U+2026 `…`. When this was changed, some of the logic
for handling different ellipsis values was left behind. It no longer
serves a purpose, so remove it.

The functions returning constants are replaced by constants. Since the
ellipsis as a `wstr` is only used in a single file, make it a local
const there and define it via the `ELLIPSIS_CHAR` const.

Put the `ELLIPSIS_CHAR` definition into `fish-widestring`, removing the
dependency of `fish-wcstringutil` on `fish-common`, helping future
extraction efforts.

One localized message contains an ellipsis, which was inserted via a
placeholder, preventing translators from localizing it. Since the
ellipsis is a constant, put it directly into the localized string.

This is done in preparation for extracting this file into its own crate.

Another step in splitting up the main library crate.

At the moment, this breaks tests outside of the new crate, since the
crate changes behavior depending on whether the `test` config is active,
but tests outside the crate get the version with `test` disabled.