fix(date): zero-pad milliseconds when formatting %N fractional seconds (#929)

%N renders the fractional part of the second. The milliseconds returned by
getMilliseconds() are the three most significant digits of that fraction and
must be zero-padded to three digits before use, otherwise sub-100ms values
lose their leading zeros:

  50ms => strftime("%N")  returned "500000000", expected "050000000"
   5ms => strftime("%3N") returned "500",       expected "005"

Pad the milliseconds to three digits before slicing to the requested width.

src/util/strftime.spec.ts

@@ -87,6 +87,15 @@ describe('util/strftime', function () {
       expect(t(time, '%10N')).toBe('1290000000')
       expect(t(time, '%0N')).toBe('129000000')
     })
+    it('should zero pad %N for sub-100ms fractional seconds', function () {
+      const time = new TestDate('2019-12-15 01:21:00.005')
+      expect(t(time, '%N')).toBe('005000000')
+      expect(t(time, '%3N')).toBe('005')
+      expect(t(time, '%6N')).toBe('005000')
+      const tens = new TestDate('2019-12-15 01:21:00.050')
+      expect(t(tens, '%N')).toBe('050000000')
+      expect(t(tens, '%2N')).toBe('05')
+    })
     it('should format %p as upper cased am/pm', function () {
       expect(t(now, '%p')).toBe('PM')
       expect(t(then, '%p')).toBe('AM')

src/util/strftime.ts

@@ -98,7 +98,7 @@ const formatCodes: Record<string, FormatCodeHandler> = {
   M: (d: LiquidDate) => d.getMinutes(),
   N: (d: LiquidDate, opts: FormatOptions) => {
     const width = Number(opts.width) || 9
-    const str = String(d.getMilliseconds()).slice(0, width)
+    const str = padStart(String(d.getMilliseconds()), 3, '0').slice(0, width)
     opts.memoryLimit?.use(width - str.length)
     return padEnd(str, width, '0')
   },