feat: add Azure Policy core JSON parser and expression parser

[anakrish]

Add the foundational parsing infrastructure for Azure Policy JSON:

• ExprParser: ARM template expression parser for "[...]" strings,
  supporting function calls, dot access, index access, and literals
• Parser (core): recursive-descent JSON tokenizer-to-AST parser that
  reads directly from Lexer tokens with no intermediate serde_json step
• ParseError: structured error types with span context for diagnostics
• Helper functions: field classification, operator kind parsing, and
  ARM template expression detection

These components are consumed by the policy-aware parsing modules
(constraint, policy_rule, policy_definition) in a subsequent PR.

[Copilot]

Pull request overview

Introduces the initial Azure Policy parsing foundation by adding span-annotated AST types plus low-level JSON and ARM-template-expression parsing components under src/languages/azure_policy/.

Changes:

• Added Azure Policy AST types (Constraint, Condition, ValueOrExpr, expression/count nodes, etc.) with Span propagation.
• Added a core token-driven JSON parser (Parser) and associated parse error types.
• Added an ARM template expression sub-parser (ExprParser) and wired new modules into azure_policy::mod.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
src/languages/azure_policy/mod.rs	Exposes new ast, expr, and parser modules from the Azure Policy language entrypoint.
src/languages/azure_policy/ast/mod.rs	Adds span-annotated AST types for policy definitions/rules/constraints/operators/fields.
src/languages/azure_policy/ast/value.rs	Adds JSON value, expression AST, and count-related node types.
src/languages/azure_policy/expr.rs	Implements an ARM template expression parser on top of the shared lexer.
src/languages/azure_policy/parser/mod.rs	Adds parser module entrypoint plus helper classification functions (fields/operators/template-expr checks).
src/languages/azure_policy/parser/core.rs	Implements the core JSON parser (Parser) and JSON→ValueOrExpr conversion helpers.
src/languages/azure_policy/parser/error.rs	Defines ParseError and its Display formatting for span-aware parsing errors.

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