feat: add postcode patterns for Colombia (CO) and British Virgin Islands (VG) (#1547)

## Summary

Two ISO 3166-1 alpha-2 country codes are present in `iso3166_1_alpha2`
(i.e., valid countries) but missing from `postCodePatternDict`, causing
`postcode_iso3166_alpha2` and `postcode_iso3166_alpha2_field` validators
to unconditionally return `false` for these countries.

This PR adds the missing patterns:

| Country | Code | Pattern | Example |
|---------|------|---------|---------|
| Colombia | `CO` | `^\d{6}$` | `110111` |
| British Virgin Islands | `VG` | `^VG\d{4}$` | `VG1110` |

### References
- Colombia postal code system (Código Postal Colombiano): 6-digit
numeric format
- BVI postal code system introduced by the BVI Postal Service: `VG`
prefix followed by 4 digits (e.g. `VG1110`, `VG1120`)

## Test plan

- All existing tests pass (`go test ./...`)
- Both patterns follow the same conventions as surrounding entries in
`postcode_regexes.go`

Author: j-ibarra

postcode_regexes.go

@@ -50,6 +50,7 @@ var postCodePatternDict = map[string]string{
 	"KH": `^\d{5}$`,
 	"CV": `^\d{4}$`,
 	"CL": `^\d{7}$`,
+	"CO": `^\d{6}$`,
 	"CR": `^\d{4,5}|\d{3}-\d{4}$`,
 	"HR": `^\d{5}$`,
 	"CY": `^\d{4}$`,
@@ -149,6 +150,7 @@ var postCodePatternDict = map[string]string{
 	"MQ": `^9[78]2\d{2}$`,
 	"NC": `^988\d{2}$`,
 	"NE": `^\d{4}$`,
+	"VG": `^VG\d{4}$`,
 	"VI": `^008(([0-4]\d)|(5[01]))([ \-]\d{4})?$`,
 	"VN": `^[0-9]{1,6}$`,
 	"PF": `^987\d{2}$`,