fix: parse identifiers with leading underscores

[joaothallis]

Problem

WhatsApp webhook payloads carry vendor keys that start with an underscore, such as _vnd. Expressions traversing them stopped parsing at the underscore:

Expression.evaluate_as_string!("@event.message._vnd.v1.chat", context)
# => "%{\"_vnd\" => %{...}}._vnd.v1.chat"
#    (the inspected message map, then the rest as literal text)

The atom combinator in Expression.Parser required the first character of an identifier to be a letter or digit, so ._vnd failed the attribute parse and the expression was truncated after event.message. These keys come from the platform's own webhook enrichment, so flow authors cannot rename them; the only workaround was bracket syntax (@event.message["_vnd"]["v1"]["chat"]).

Fix

Allow leading underscores in the atom grammar while still requiring at least one letter or digit, so a bare _ keeps failing to parse (@(_) remains literal text, pinned by an existing test).

Behavior changes

Input	Before	After
@event.message._vnd.v1.chat	truncated after event.message, tail rendered as literal text	resolves the full chain
@_missing (not in context)	rendered _missing — the @ was silently swallowed by an empty expression match	rendered @_missing, the documented round-trip for unresolved variables
@(_)	literal, no expression parsed	unchanged

Tests

• Parser: @_foo / @__foo as variables, @event.message._vnd.v1.chat attribute chain, @(foo._bar) block form
• Evaluation: resolving through _vnd-style nested keys as string and block, unresolved @_missing round-trip
• Full suite: 688 passed, mix format and mix credo --strict clean

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