build(W-23983692): upgrade to typescript v6 and eslint v10

[k80bowman]

What does this PR do?

Migrates the CLI's dev toolchain to TypeScript v6 and eslint v10:

1. Replaces the legacy .eslintrc.cjs files with an ESLint 10 flat config (eslint.config.mjs), removing .eslintignore, root .eslintrc.cjs, and test/.eslintrc.cjs.
2. Bumps @salesforce/dev-scripts to v14, eslint to v10, typescript to v6, and @salesforce/ts-types to ^3.2.0.
3. Adds explicit @types/* devDeps (chai, mocha, node, sinon) and mocha types in tsconfig.json/test/tsconfig.json now required by the toolchain, plus a rootDir and the **/eslint.config.* glob to the wireit lint inputs.

The only source change is in src/flags.ts, applying the ??= idiom now enforced by the updated lint config.

Acceptance Criteria

• yarn build, yarn lint, and yarn test pass under TypeScript 6, ESLint 10 flat config, and dev-scripts 14.

Testing Notes

Covered by the existing automated build/lint/test suite. Verify locally with a clean install:

1. yarn install
2. yarn build
3. yarn lint — confirm the flat config is picked up and there are no errors.
4. yarn test

No manual CLI testing applicable — there are no command, flag, or output changes.

Checklist

• This change has been approved by the CLI Review Board or approval isn't required. Anything that adds/changes/deletes commands, flags, output, or json output has to be reviewed.
• Does this follow the deprecation policy?

What issues does this PR fix or reference?

@W-23983692@

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