build: migrate eslint config to flat style (#913)

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## Description

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Migrate .eslintrc to esling.config.mjs. Using .mjs so I can use `import`
(.ts configs are still experimental). Fixed a few lint issues that were
easy, changed one new lint rule to a warn since it wasn't caught before.

## Related Issue

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Fixes #899 

## Motivation and Context

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Using the new flat config for Eslint is required for upgrading to 9.0

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Ran `npm run lint` and VS Code Eslint plugin many times

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- [ ] Bug fix (non-breaking change which fixes an issue)
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- [ ] Breaking change (fix or feature that would cause existing
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- [x] My code follows the code style of this project.
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eslint.config.mjs

@@ -0,0 +1,103 @@
+import eslintPluginPrettierRecommended from 'eslint-plugin-prettier/recommended';
+import tseslint from 'typescript-eslint';
+import globals from 'globals';
+import tsParser from '@typescript-eslint/parser';
+import jest from 'eslint-plugin-jest';
+import js from '@eslint/js';
+
+export default [
+	// using .mjs for module support as TypeScript Eslint configs are still experimental
+	{
+		name: 'Eslint config file',
+		files: ['eslint.config.mjs'],
+		// ...tseslint.configs.disableTypeChecked,
+		// ...js.configs.recommended,
+
+		languageOptions: {
+			sourceType: 'module',
+			parserOptions: {
+				// project: 'package.json'
+			}
+		}
+	},
+
+	{
+		name: 'general project rules',
+		files: ['**/*.ts'],
+
+		plugins: {
+			'@typescript-eslint': tseslint.plugin
+		},
+
+		languageOptions: {
+			globals: {
+				...globals.node
+			},
+
+			parser: tsParser,
+			ecmaVersion: 5,
+			sourceType: 'module',
+
+			parserOptions: {
+				project: 'tsconfig.json'
+			}
+		},
+
+		rules: {
+			// contains all of recommended, recommended-type-checked, and strict
+			...tseslint.configs.strictTypeChecked.rules,
+			'@typescript-eslint/no-unused-vars': [
+				'warn',
+				{
+					argsIgnorePattern: '^_'
+				}
+			],
+
+			'@typescript-eslint/naming-convention': [
+				'warn',
+				{
+					selector: ['typeLike'],
+					format: ['PascalCase']
+				},
+				{
+					selector: ['variableLike', 'function'],
+					format: ['camelCase'],
+					leadingUnderscore: 'allow'
+				},
+				{
+					selector: ['variable'],
+					format: ['camelCase', 'UPPER_CASE']
+				},
+				{
+					selector: 'variable',
+					types: ['boolean'],
+					format: ['PascalCase'],
+					prefix: ['is', 'should', 'has', 'can', 'did', 'was', 'will']
+				}
+			],
+			// this is set to warn because it wasn't caught before the eslint migration in 11/2024
+			'@typescript-eslint/restrict-template-expressions': 'warn'
+		}
+	},
+
+	{
+		name: 'test rules',
+		files: ['tests/**/*.ts'],
+
+		...jest.configs['flat/recommended'],
+		...jest.configs['flat/style'],
+
+		plugins: {
+			jest
+		},
+
+		rules: {
+			'@typescript-eslint/no-empty-function': 'off',
+			'@typescript-eslint/unbound-method': 'off',
+			'jest/no-done-callback': 'off'
+		}
+	},
+
+	// Prettier plugin which should be last
+	eslintPluginPrettierRecommended
+];