test: Enable test for explicit-function-return-type rule (#168)

## Summary

This PR enables the test for the `explicit-function-return-type` rule
by:
1. Uncommenting the test file in `rstest.config.mts` (line 47)
2. Implementing the rule from scratch in Go
3. Registering the rule in the global rule registry

## Implementation Details

The `explicit-function-return-type` rule enforces explicit return type
annotations on functions and class methods in TypeScript code. This
improves code clarity and can enhance type-checking performance in large
codebases.

### Supported Configuration Options

- `allowExpressions` (default: `false`) - Permits function expressions
to skip return type declarations
- `allowTypedFunctionExpressions` (default: `true`) - Ignores functions
with type annotations on the variable assignment
- `allowHigherOrderFunctions` (default: `true`) - Overlooks functions
that directly return other functions
- `allowDirectConstAssertionInArrowFunctions` (default: `true`) -
Exempts arrow functions using `as const` assertions
- `allowConciseArrowFunctionExpressionsStartingWithVoid` (default:
`false`) - Allows arrow functions prefixed with `void` keyword
- `allowFunctionsWithoutTypeParameters` (default: `false`) - Ignores
non-generic functions
- `allowIIFEs` (default: `false`) - Exempts immediately invoked function
expressions
- `allowedNames` (default: `[]`) - Array of specific function/method
names to ignore

### Files Changed

- `packages/rslint-test-tools/rstest.config.mts` - Uncommented test file
-
`internal/plugins/typescript/rules/explicit_function_return_type/explicit_function_return_type.go`
- New rule implementation
- `internal/config/config.go` - Rule registration

## Testing

The implementation follows the TypeScript-ESLint specification and
handles:
- Function declarations
- Function expressions
- Arrow functions
- Method declarations
- Getter accessors
- All configuration options from the spec

## References

- TypeScript-ESLint rule documentation:
https://typescript-eslint.io/rules/explicit-function-return-type/
- Test file:
`packages/rslint-test-tools/tests/typescript-eslint/rules/explicit-function-return-type.test.ts`

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: claude[bot] <41898282+claude[bot]@users.noreply.github.com>
Co-authored-by: Claude <noreply@anthropic.com>

Author: 3 people

internal/config/config.go

@@ -26,6 +26,7 @@ import (
 	"github.com/web-infra-dev/rslint/internal/plugins/typescript/rules/consistent_type_exports"
 	"github.com/web-infra-dev/rslint/internal/plugins/typescript/rules/consistent_type_imports"
 	"github.com/web-infra-dev/rslint/internal/plugins/typescript/rules/default_param_last"
+	"github.com/web-infra-dev/rslint/internal/plugins/typescript/rules/explicit_function_return_type"
 	"github.com/web-infra-dev/rslint/internal/plugins/typescript/rules/no_array_delete"
 	"github.com/web-infra-dev/rslint/internal/plugins/typescript/rules/no_base_to_string"
 	"github.com/web-infra-dev/rslint/internal/plugins/typescript/rules/no_confusing_void_expression"
@@ -380,6 +381,7 @@ func registerAllTypeScriptEslintPluginRules() {
 	GlobalRuleRegistry.Register("@typescript-eslint/consistent-type-imports", consistent_type_imports.ConsistentTypeImportsRule)
 	GlobalRuleRegistry.Register("@typescript-eslint/default-param-last", default_param_last.DefaultParamLastRule)
 	GlobalRuleRegistry.Register("@typescript-eslint/dot-notation", dot_notation.DotNotationRule)
+	GlobalRuleRegistry.Register("@typescript-eslint/explicit-function-return-type", explicit_function_return_type.ExplicitFunctionReturnTypeRule)
 	GlobalRuleRegistry.Register("@typescript-eslint/no-array-delete", no_array_delete.NoArrayDeleteRule)
 	GlobalRuleRegistry.Register("@typescript-eslint/no-base-to-string", no_base_to_string.NoBaseToStringRule)
 	GlobalRuleRegistry.Register("@typescript-eslint/no-confusing-void-expression", no_confusing_void_expression.NoConfusingVoidExpressionRule)