Re-design the CLI interface

[jessebraham]

This PR includes a re-design of the command-line interface, most notably adding a flash subcommand.

In the current interface, since there is no flashing subcommand, it's possible to pass arguments before a subcommand which are accepted but silently ignored. This is bad UX and is a common source of confusion for users. Adding a flashing subcommand eliminates this issue entirely.

While I was breaking things, I did a bit of refactoring as well. This was mostly just moving types to the module they belong in, simplifying some functions, etc. Nothing too interesting. I've also renamed a number of types/fields/functions/etc. to be more in line with other parts of the code, eg.) CLI arguments now use arg instead of opt, as their types are even clap::Arg.

Additionally I have created a cli feature which all CLI-related dependencies are gated behind. This allows these packages to be excluded when using espflash as a library. This feature is enabled by default. The executable was renamed from main.rs to bin/espflash.rs to accommodate this.

Beyond addressing whatever changes are requested, I'm not sure that there is much more to add to this PR. There are additional UX-related changes I would like to make but they are out of scope and deserve their own PRs, I think.

I would like to spend some time reviewing this and really picking it apart before we merge it. I have marked this as a draft until I am comfortable with its state. Small details such as short/long names, docstrings (which are displayed in help messages) and argument names are important, so please nitpick!

Closes #222
Closes #236

[hnidoaht-101]

hi @jessebraham I come here via esp-rs chat channel. Its clear that I'm totally newbie, so please forgive if I have some naive questions 😅. Is there any reasons you decided to switch from tracing-subcriber crate to env-logger?

[jessebraham]

@hnidoaht-101 for context, tracing-subscriber was added in a recent PR, we did not have any logging prior and I just really didn't scrutinize this inclusion (partially because I was already working on revamping the CLI anyways, so it didn't really matter much).

env-logger is what I would normally use in these situations, and it's a much simpler package. I frankly had never even heard of tracing-subscriber before that PR, so if there is some tangible benefit in using it then I would consider reverting, but I'm not sure what said benefit(s) would be.

[bjoernQ]

Haven't looked much into this yet but definitely thumbs-up for adding a flash command and the other changes described here

[bjoernQ]

LGTM - also I'm using it for a few days without issues

Maybe we should bump the version in this PR already? No needed but I think it would be nice

[jessebraham]

Thanks for taking a look!

I wasn't sure when to bump the version, I can set it to 2.0.0-dev for now though I suppose. I do plan on submitting a handful more PRs prior to releasing (mostly just refactoring, but now is the time to break public APIs if we're gonna do it!)