Initial support for external v8 strings

[AaronO]

This PR partially fixes #636, by adding support for external static rust strings.

Rust doesn't have utf16 (or two-byte string) literals, so we could shorten the name in this static case, i.e:

v8::String::new_external_static()
instead of
v8::String::new_external_onebyte_static()

We could support non-static external strings too, but I'm not sure we want to encourage that.

[ry]

Looks good. Could you add a small test?

I'd prefer to keep the function name the same as in V8, as you have done.

[AaronO]

@ry Added some simple tests and exposed some relevant v8 methods:

is_external()
is_external_onebyte()
is_external_twobyte()
is_onebyte()

See notes on is_external(), it needs to be updated on next v8 release, since v8::String::IsExternal() was only recently re-added (https://source.chromium.org/chromium/_/chromium/v8/v8.git/+/1dd8624b524d14076160c1743f7da0b20fbe68e0).

The C++ binding fallsback to v8::Value::IsExternal() which is incorrect, the rust fallback I added is perfectly functional however, so this can ship without updating v8

[ry]

LGTM - thanks @AaronO