Merge bitcoindevkit/bdk#1946: chore: create and apply rustfmt.toml

7bdbcdd2a1b6bb0fb9fafd7792615a1a3a302a5a chore: apply fmt (Luis Schwab)
655b77b4ddc8982c3c747f5553abb83801b09a0b chore: create rustfmt.toml, update CI to use nightly for fmt job (Luis Schwab)

Pull request description:

  This PR adds basic formatting configuration for uniform comments. Note that the nightly toolchain is necessary.

  ### Changelog:

  - Create rustfmt.toml with this config:
  ```
  comment_width = 100
  format_code_in_doc_comments = true
  wrap_comments = true
  ```
  - Apply rustfmt.toml.
  - Update the fmt CI job to use the nightly toolchain.
  - Update PR template to use nightly toolchain.

  Note: I had to add #[rustfmt::skip] to test_extract_satisfaction_timelock() because the base64 PSBT would get wrapped due to the slashes.

  #### 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 +nightly fmt` and `cargo clippy` before committing

ACKs for top commit:
  notmandatory:
    ACK 7bdbcdd2a1b6bb0fb9fafd7792615a1a3a302a5a

Tree-SHA512: 03b056e1da5733e8aa9a9244192b48a5c170902216d936ddbeaf81e22608fee3344ae6386a06019c7806f0a3e1597ab5be34f78661809181693c63954a61cce0