fix: detect the dual license and expand the pub.dev description

[Johnosezele]

Summary

• Put the Apache-2.0 and MIT texts in LICENSE. Pana only scores that filename, and the old pointer was detected as an unknown license. LICENSE-MIT and LICENSE-APACHE are unchanged.
• Expand the pubspec.yaml description to 110 characters so it says this is for Dart and Flutter descriptor wallets. Pana wants 50–180 characters.

Test

• LICENSE is still MIT OR Apache-2.0, with both full texts in that file
• The pubspec description is between 50 and 180 characters

[reez]

General idea of upping our score is good, but our current LICENSE matches upstream bdk, bdk_wallet, and bdk-ffi, including “Some files retain their own copyright notice” and “Except as otherwise noted in individual files”, but this rewrite drops those clauses.

[Johnosezele]

I shall add them, thanks for pointing it out.

[Johnosezele]

General idea of upping our score is good

It will also improve the library credibility as an official one.

but our current LICENSE matches upstream bdk, bdk_wallet, and bdk-ffi,

true, the problem lies here because the underlying check for pub.dev checks for only LICENSE and expects to find a valid MIT or APACHE one in there, but since we have it in separate files, its not able to recognize.

@reez I've inlined the two full license texts into LICENSE, plus the required wordings from upstream LICENSE.