chore: create and apply rustfmt.toml

[luisschwab]

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:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

[GideonBature]

tACK #1946

Can you please squash all three commit messages as a single commit?

[oleonardolima]

cACK 396fff8

[evanlinjin]

Is it some sort of standard to have comment_width = 140?

Currently, most of the documentation is kept within 100.

[luisschwab]

Is it some sort of standard to have comment_width = 140?

Not really,. Either one is fine for me.

[evanlinjin]

In that case, I would prefer to stick with 100. It means you can see more if you use a split screen.

[luisschwab]

I made an identical PR on bdk_wallet, once this get's merged I'll update it.