fix(compiler): strip abstract keyword from class expressions in JIT mode

[ashley-hunter]

Closes #148

Summary

• Fixes JIT compilation of abstract classes by stripping the abstract keyword when converting class declarations to class expressions (let X = class X)
• Class expressions in JavaScript/TypeScript cannot use the abstract modifier, so it must be removed during the JIT transform

Test plan

• Added integration test test_jit_abstract_class covering exported abstract class with @Injectable()
• Verifies abstract class does not appear in output
• Verifies correct class expression form (let X = class X)
• Verifies __decorate call is present
• Snapshot test for full output validation