core: implement flush and poll

[joshua-holmes]

Triggers the following error:

Segmentation fault at address 0x520
/home/josh/zig/0.14.0-dev.1911+3bf89f55c/files/lib/std/fifo.zig:245:24: 0x134f3e8 in writableLength (core-triangle)
            return self.buf.len - self.count;
                       ^
/home/josh/zig/0.14.0-dev.1911+3bf89f55c/files/lib/std/fifo.zig:136:36: 0x125e015 in ensureUnusedCapacity (core-triangle)
            if (self.writableLength() >= size) return;
                                   ^
/home/josh/zig/0.14.0-dev.1911+3bf89f55c/files/lib/std/fifo.zig:299:42: 0x125df76 in writeItem (core-triangle)
            try self.ensureUnusedCapacity(1);
                                         ^
/home/josh/dev/open-source/mach/src/Core.zig:379:26: 0x13763b7 in keyboardHandleEnter (core-triangle)
    core.events.writeItem(event) catch {
                         ^
???:?:?: 0x7d27dc80a595 in ??? (libffi.so.8)
Unwind information for `libffi.so.8:0x7d27dc80a595` was not available, trace may be incomplete

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