fix: add isModule property to Node for script type=module detection

[maruthang]

Summary

Fixes #133

Bug: Module scripts (<script type="module">) were not distinguishable from classic scripts, causing consumers to incorrectly report "Cannot redeclare block-scoped variable" errors when multiple module scripts each declare the same variable.

Root Cause: The Node class and interface had no way to indicate whether a <script> tag uses type="module", so all scripts were treated as sharing a single global scope.

Fix: Added an isModule computed property to the Node class and interface that returns true when the node is a <script> tag with type="module", allowing consumers to treat each module script as having its own isolated scope per the HTML specification.

Changes

• src/htmlLanguageTypes.ts: Added readonly isModule: boolean to the Node interface with JSDoc documentation.
• src/parser/htmlParser.ts: Implemented isModule as a getter on the Node class that checks the tag is script and the type attribute value (unquoted) equals module.
• src/test/parser.test.ts: Added 7 regression tests covering double-quoted, single-quoted, classic scripts, non-script tags, multiple module scripts, and mixed module/classic scripts.

Testing

• Added regression tests in src/test/parser.test.ts that verify isModule returns the correct value for all relevant scenarios.
• All 156 existing tests pass.