feat(core): implement Deno.core.isProxy()

[AaronO]

Another binding ... but it's required for #12286 and could be useful elsewhere

core.getProxyDetails(...) isn't fast enough for fast-path checks:

No fast path:
resp_w_bh:               n = 1000000, dt = 1.295s, r = 772201/s, t = 1294ns/op
Fast path with getProxyDetails check:
resp_w_bh:               n = 1000000, dt = 1.321s, r = 757002/s, t = 1321ns/op
Fast path with isProxy check:
resp_w_bh:               n = 1000000, dt = 1.146s, r = 872600/s, t = 1146ns/op
Forced fast path:
resp_w_bh:               n = 1000000, dt = 1.112s, r = 899281/s, t = 1112ns/op

[bartlomieju]

Looks ok, but can you add a test before landing?