Node: use 'Native' to refer to the loaded Rust library

Specifically, use 'NativeImpl' to refer to the run-time bindings
(instead of 'SC'), and 'Native' to refer to the TypeScript module for
those bindings (instead of 'SignalClient'). This makes
compile-time diagnostics clearer, since the overall Node package is
named 'signal-client'.

.prettierignore

@@ -1,2 +1,2 @@
 node/dist/**
-node/libsignal_client.d.ts
+node/Native.d.ts

rust/bridge/node/bin/gen_ts_decl.py

@@ -158,7 +158,7 @@ decls = itertools.chain(
     collect_decls(os.path.join(our_abs_dir, '..')),
     collect_decls(os.path.join(our_abs_dir, '..', '..', 'shared'), features='node'))
 
-output_file_name = 'libsignal_client.d.ts'
+output_file_name = 'Native.d.ts'
 contents = open(os.path.join(our_abs_dir, output_file_name + '.in')).read()
 contents += "\n"
 contents += "\n".join(sorted(decls))