return a disposable string

[krichprollsch]

When the user's API return a not owned []const u8, jsruntime must free the string once v8 copied it.
We must cleary express the two differents strings returned by the user's API:

• string owned by the user's API must continue to return []const u8. This string must not be free by jsruntime
• string owned by the caller must return a new jsruntime alias of []const u8. jsruntime must free this string.

Name ideas for the alias: TmpStr,ShortLiveStr, PassingStr, FreeStr, DisposableStr...