Change month microformat from 'm' to 'mo' in narrow style

Co-authored-by: francinelucca <40550942+francinelucca@users.noreply.github.com>

Author: Copilot

README.md

@@ -273,7 +273,7 @@ This will used to determine the length of the unit names. This value is passed t
 | relative   | narrow         | in 1 mo.                 |
 | duration   | long           | 1 month, 2 days, 4 hours |
 | duration   | short          | 1 mth, 2 days, 4 hr      |
-| duration   | narrow         | 1m 2d 4h                 |
+| duration   | narrow         | 1mo 2d 4h                |
 
 ##### second, minute, hour, weekday, day, month, year, timeZoneName
 

src/duration-format-ponyfill.ts

@@ -110,7 +110,15 @@ export default class DurationFormat {
           : unitStyle === 'numeric'
           ? {}
           : {style: 'unit', unit: nfUnit, unitDisplay: unitStyle}
-      list.push(new Intl.NumberFormat(locale, nfOpts).format(value))
+      
+      let formattedValue = new Intl.NumberFormat(locale, nfOpts).format(value)
+      
+      // Custom handling for narrow month formatting to use "mo" instead of "m"
+      if (unit === 'months' && unitStyle === 'narrow') {
+        formattedValue = formattedValue.replace(/(\d+)m$/, '$1mo')
+      }
+      
+      list.push(formattedValue)
     }
     return new ListFormat(locale, {
       type: 'unit',

test/duration-format-ponyfill.ts

@@ -37,7 +37,7 @@ suite('duration format ponyfill', function () {
       locale: 'en',
       style: 'narrow',
       parts: [
-        {type: 'element', value: '1m'},
+        {type: 'element', value: '1mo'},
         {type: 'literal', value: ' '},
         {type: 'element', value: '2d'},
         {type: 'literal', value: ' '},

test/relative-time.js

@@ -1941,7 +1941,7 @@ suite('relative-time', function () {
         datetime: '2022-09-24T14:46:00.000Z',
         tense: 'future',
         format: 'micro',
-        expected: '1m',
+        expected: '1mo',
       },
       {
         datetime: '2022-10-23T14:46:00.000Z',
@@ -1991,7 +1991,7 @@ suite('relative-time', function () {
         datetime: '2022-11-24T14:46:00.000Z',
         tense: 'future',
         format: 'micro',
-        expected: '1m',
+        expected: '1mo',
       },
       {
         datetime: '2023-10-23T14:46:00.000Z',
@@ -2023,7 +2023,7 @@ suite('relative-time', function () {
         datetime: '2022-11-24T14:46:00.000Z',
         tense: 'past',
         format: 'micro',
-        expected: '1m',
+        expected: '1mo',
       },
       {
         datetime: '2022-10-25T14:46:00.000Z',
@@ -2073,7 +2073,7 @@ suite('relative-time', function () {
         datetime: '2022-09-23T14:46:00.000Z',
         tense: 'past',
         format: 'micro',
-        expected: '1m',
+        expected: '1mo',
       },
       {
         datetime: '2021-10-25T14:46:00.000Z',
@@ -2178,13 +2178,13 @@ suite('relative-time', function () {
       {
         datetime: '2021-10-30T14:46:00.000Z',
         format: 'elapsed',
-        expected: '11m 29d',
+        expected: '11mo 29d',
       },
       {
         datetime: '2021-10-30T14:46:00.000Z',
         format: 'elapsed',
         precision: 'month',
-        expected: '11m',
+        expected: '11mo',
       },
       {
         datetime: '2021-10-29T14:46:00.000Z',