Merge #215: Chore: migrate library version naming convention on master

1022fce chore: migrate library version naming convention on master (thunderbiscuit)

Pull request description:

  This PR implements the discussion started on #19, as well as discussions over a few dev calls.

  The preferred approach is the one used by [rust-bitcoin](https://github.com/rust-bitcoin/rust-bitcoin/blob/master/bitcoin/Cargo.toml).

  ### Notes to the reviewers

  The core workflow for this is to keep the version on the master branch always an `alpha.0`, and simply update it to the next planned release once a branch has been created for a release.

  So for example, the master branch could be on `1.3.0-alpha.0` until the branch for 1.3.0 is cut, at which moment the master moves on to `1.4.0-alpha.0`. Someone pulling the master branch is always building one of those `-alpha.0` versions.

  ### Changelog notice

  None.

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

ACKs for top commit:
  oleonardolima:
    ACK 1022fce
  notmandatory:
    ACK 1022fce

Tree-SHA512: d1758387169569ec3e366e76bb9548197efd0249c9d6047112e2501ce8068d1edcbe2bf491e92ae41e4dda45ef1c286c67a3a2d0612acc9010fe63dc0ee3c359

Author: notmandatory

wallet/Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 name = "bdk_wallet"
 homepage = "https://bitcoindevkit.org"
-version = "1.2.0"
+version = "2.0.0-alpha.0"
 repository = "https://github.com/bitcoindevkit/bdk_wallet"
 documentation = "https://docs.rs/bdk_wallet"
 description = "A modern, lightweight, descriptor-based wallet library"