fix: Pad milliseconds with three zeroes

[bmaupin]

Hi!

This fork appears to have inherited a bug from upstream. As this seems to be the best-maintained fork of glossy, I'm submitting this PR to fix this issue but also in hopes that you might be interested in publishing this to NPM for others to use 😀

The bug is that milliseconds are padded only with two zeroes, e.g.

<131>1 2021-09-27T15:52:35.35-07:00 localhost test-graylog-syslog 103065 - - error: Test message

Some logging platforms (like Graylog, not sure about logdna) parse 15:52:35.35 as 15:52:35.350 instead of 15:52:35.035, so this fixes that.

Here's a related issue with more details: winstonjs/winston-syslog#139

Thanks!

[bmaupin]

I also just ran across this in the spec in case there's any doubt about this being a bug:

The TIMESTAMP described in Section 6.2.3 supports fractional seconds.
This provides grounds for a very common coding error, where leading
zeros are removed from the fractional seconds. For example, the
TIMESTAMP "2003-10-11T22:13:14.003" may be erroneously written as
"2003-10-11T22:13:14.3". This would indicate 300 milliseconds
instead of the 3 milliseconds actually meant.

(https://datatracker.ietf.org/doc/html/rfc5424#appendix-A.4)

Thanks!