Add ISO-3166-1 compliant "GB" alpha-2 code for United Kingdom

[Copilot]

Flagpack currently only supports "GB-UKM" (subdivision code) and "GBR" (alpha-3) for the United Kingdom. Per ISO-3166-1, "GB" is the standard alpha-2 code and was not resolving.

Changes

• Code lists: Added GB entry with alpha3: "GBR", numeric: "836" to src/countryCodeList.ts and countryCodeList.json, positioned before subdivision codes for lookup priority
• SVG sources: Replaced svg/{s,m,l}/GB.svg with Union Jack (previously contained England flag)
• Distribution: Added lib/flags/{s,m,l}/{GB,GBR,836}.svg and compiled TypeScript

Backward Compatibility

Existing codes continue to work:

• GB-UKM → United Kingdom (legacy)
• GB-ENG, GB-SCT, GB-WLS, GB-NIR → Subdivision flags

Result

// Now works per ISO-3166-1
isoToCountryCode('GB')              // "GB"
isoToCountryCode('GB', 'alpha3')    // "GBR"
isoToCountryCode('GBR')             // "GB"

// Usage in components
<Flag code="GB" />   // ✓ Now displays UK flag
<Flag code="GBR" />  // ✓ Works

Original prompt

Please make a PR that fixes this issue, we should support this case
According to the ISO-3166-1 standard, the alpha-2 country code for the United Kingdom is “GB”, and the alpha-3 code is “GBR”. However, Flagpack does not resolve the alpha-2 code “GB” at all, while “GBR” works correctly.
On your website’s country list, the “2-alpha“ column does not contain the expected ISO-3166-1 alpha-2 code “GB”. Instead, it shows ISO-3166-2 subdivision codes such as:
GB-UKM (United Kingdom)
GB-ENG (England)
GB-SCT (Scotland)
GB-WLS (Wales)
GB-NIR (Northern Ireland)
As a result, calling the component like this:

returns no flag, but:

works correctly.
Could you please confirm whether this is the intended behaviour?
If “GB” is expected to be supported as the ISO alpha-2 code, it would be helpful to add a direct mapping or clarify the documentation. Currently the “2-alpha” column appears to list subdivision codes, which makes the behaviour unclear.

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