fix: parse identifiers with leading underscores

[santiagocardo]

Problem

Some payloads may have keys that start with an underscore, such as _vnd. Expressions traversing them stop 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 requires the first character of an identifier to be a letter or digit, so ._vnd fails the attribute parse, and the expression is truncated after event.message.

Fix

Allow leading underscores in the atom grammar while still requiring at least one letter or digit, so a bare _ keeps failing to parse.

atom =
  ascii_string([?_], min: 0)
  |> ascii_string([?a..?z, ?A..?Z, ?0..?9], min: 1)
  |> ascii_string([?a..?z, ?A..?Z, ?0..?9, ?_, ?-], min: 0)

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

The @(_) row matters: keeping the min: 1 letter-or-digit requirement is what stops a bare underscore becoming an identifier, so the existing guard test stays green and Elixir's "underscore means unused" convention doesn't bleed into map keys.

This supersedes #369, which was closed unmerged while still a draft.