Implement JEP-18 let expression with lexical scoping

[joshrotenberg]

This PR implements JEP-18 (Lexical Scoping) as an optional feature. Let expressions allow binding values to variables for use within an expression, enabling queries that reference elements from outer scopes.

Syntax

let $variable = expression in body
let $var1 = expr1, $var2 = expr2 in body

Example

// Bind a threshold and use it in a filter
let expr = jmespath::compile("let $threshold = `50` in numbers[? @ > $threshold]").unwrap();

// Reference parent data within a nested filter
let expr = jmespath::compile(
    "[*].[let $home_state = home_state in states[? name == $home_state].cities[]][]"
).unwrap();

Features

Per JEP-18 specification:

• let and in are contextual keywords (not reserved)
• Variable references use $name syntax
• Multiple comma-separated bindings supported
• Lexical scoping with inner scope shadowing
• Bindings evaluated in outer scope before body executes
• Projection stopping when bound to variable
• Undefined variable triggers runtime error

Feature-gated behind let-expr to maintain backward compatibility:

[dependencies]
jmespath = { version = "0.5", features = ["let-expr"] }

Changes

• Add let-expr feature flag to Cargo.toml
• Add Variable and Assign tokens to lexer (feature-gated)
• Add VariableRef and Let AST nodes
• Implement let expression parsing in parser
• Add scope stack to Context for variable lookup
• Implement Let and VariableRef evaluation in interpreter
• Add rustdoc documentation with examples

Testing

• All 861 compliance tests pass
• 34 unit tests covering:
  • Basic bindings, multiple bindings, nested scopes
  • Variable shadowing and scope restoration
  • Projection stopping behavior
  • Variables in filters, functions, pipes, multiselect
  • Boolean operators (&&, ||, !)
  • Flatten, slice operations
  • Error cases (undefined variable, out of scope, syntax errors)

Known Limitations

Using let or in as field names immediately before the in keyword (e.g., let $x = foo.let in ...) requires more complex parser lookahead. Workaround: use quoted identifiers (let $x = foo."let" in ...).