Add additional identification guards to Sourceror.Identifier

[zachallaun]

Implements the lower-level parts of #105.

This PR adds the following guards:

• is_call/1
• is_unqualified_call/1
• is_qualified_call/1
• is_identifier/1
• is_atomic_literal/1

Additional changes/fixes:

• Add the power operator (**) to the list of binary ops for Elixir 1.13+
• Refactor SourcerorTest.Support.Corpus somewhat to make its use a bit more ergonomic
• Remove a bit of leftover tree-sitter syntax from test/corpus/term/alias.ex (it wasn't erroring because S-expressions are also valid Elixir)

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While working on this, I realized that these lower-level primitives might prove sufficient for the vast majority of cases. The higher-level things discussed in #105 are likely still useful, but I'd kind of like to kick the tires on these changes before introducing additional new API.

[doorgan]

Great job! The changes look great 💜

While working on this, I realized that these lower-level primitives might prove sufficient for the vast majority of cases. The higher-level things discussed in #105 are likely still useful, but I'd kind of like to kick the tires on these changes before introducing additional new API.

Agreed! I'm a fan of building bottom up, so let's see how these changes work before adding more on top 😄