Show syntax macro completions when Kernel. is prefixed to the cursor.

[Moosieus]

Closes #832.

This solution is imperfect because it requires the prefix to end exactly with Kernel.

However it works in most cases because:

• Completions are usually requested at the point of Kernel.|
• Completions are cached by clients for the next few characters.

In the exceptional rare case where a user copy/pastes something like Kernel.=| and requests a completion, these won't show. Not a perfectly correct solution, but our handling of syntax macros is already a matter of taste, so ¯\_(ツ)_/¯