Improved commands sub-system

[aperepel]

Reworked the command subsystem. Commands are now implemented as traits with default
template methods and just need to fill out the customized parts (vs duplicating most of the code).

Improved the user feedback, made the output consistent across commands.
Fixed error handling so any download and binary executable resolution problems are exposed (vs being silently swallowed).

Some Highlights:

• cargo leptos will now check (once a day) for new versions of tools like sass,
  tailwindcss, wasm-ops via the GitHub release API and inform the user of a newer available version. This happens lazily
  for commands when they are used, but only once on startup and no more frequently than once a day.
• As part of the above exercise, improved the version recognition, so all kinds of variants like 1.0, version_112, v3.3.1, etc are consistently handled and compared. This lays the foundation for the semver style version requests.
• As prompted by the command, a user can quickly try out the new version by overriding the env var, e.g. LEPTOS_TAILWIND_VERSION=v3.3.3 cargo leptos watch --hot-reload -vv. The binary will
  be downloaded transparently and cached. Switching between tool versions has never been easier!
• Naturally, downgrades are now trivial as well (just pin the version via the env var).

Added the following env vars:

LEPTOS_CARGO_GENERATE_VERSION
LEPTOS_TAILWIND_VERSION
LEPTOS_SASS_VERSION
LEPTOS_WASM_OPT_VERSION

Things I'd Like to Do:

• Switch to the semver style requests, so e.g. a patch-compatible version can be declared as desired and the updates are happening easily and transparently.
• Related to the above, remove the need for a hardcoded default version for every tool (do the best thing OOTB). Leveraging semver will ensure we pick up only compatible upgrades, unless instructed otherwise by the user.

P.S.: I'm not claiming to have extensive idiomatic Rust knowledge, feel free to propose all kinds of improvements to the code style.