Add simplex install <dep> functionality

[LesterEvSe]

• This PR suggests a bug fix and I've added the necessary tests.
• This PR introduces a new feature and I've discussed the update in an Issue or with the team.
• This PR is just a minor change like a typo fix.

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Adds support for installing dependencies directly from the CLI. Dependencies can now be added to Simplex.toml via:

simplex install <alias>=<source>

The alias can be omitted, in which case it is derived from the last path segment of the source. For example:

simplex install https://github.com/BlockstreamResearch/simplicityhl-std
# -> simplicityhl-std = { git = "https://github.com/BlockstreamResearch/simplicityhl-std" }

Multiple dependencies can be added in a single invocation, and the two forms can be freely mixed:

simplex install merkle=deps/merkle base_math=deps/math https://github.com/BlockstreamResearch/simplicityhl-std

[Arvolear]

LGTM!