nip01: add support for filter prefix in authors and ids

so clients can filter events by prefix in authors and ids as
described in nip-01, i.e. to subscribe to mined events starting
with zeroes or to add some privacy for clients that may not want
to disclose the exact filter.

see also https://github.com/scsibug/nostr-rs-relay/issues/104

filter.test.js

@@ -37,6 +37,16 @@ describe('Filter', () => {
       expect(result).toEqual(false)
     })
 
+    it('should return true when the event id starts with a prefix', () => {
+      const filter = {ids: ['22', '00']}
+
+      const event = {id: '001'}
+
+      const result = matchFilter(filter, event)
+
+      expect(result).toEqual(true)
+    })
+
     it('should return false when the event kind is not in the filter', () => {
       const filter = {kinds: [1, 2, 3]}
 
@@ -132,6 +142,20 @@ describe('Filter', () => {
       expect(result).toEqual(true)
     })
 
+    it('should return true when at least one prefix matches the event', () => {
+      const filters = [
+        {ids: ['1'], kinds: [1], authors: ['a']},
+        {ids: ['4'], kinds: [2], authors: ['d']},
+        {ids: ['9'], kinds: [3], authors: ['g']}
+      ]
+
+      const event = {id: '987', kind: 3, pubkey: 'ghi'}
+
+      const result = matchFilters(filters, event)
+
+      expect(result).toEqual(true)
+    })
+
     it('should return true when event matches one or more filters and some have limit set', () => {
       const filters = [
         {ids: ['123'], limit: 1},