fix: add exports field to package.json for CSS subpath resolution

[renevanpelt]

Problem

The published @xterm/xterm package has no exports field in its package.json. In strict module resolution environments — such as pnpm workspaces with moduleResolution: bundler or node16 — subpath imports like:

import '@xterm/xterm/css/xterm.css'

fail with an error along the lines of "Package subpath './css/xterm.css' is not defined by 'exports'".

Fix

Add an exports map that exposes:

• "." — the existing main entry (./lib/xterm.js) for both require and import
• "./css/xterm.css" — the stylesheet, so CSS subpath imports resolve correctly

"exports": {
  ".": {
    "require": "./lib/xterm.js",
    "import": "./lib/xterm.js",
    "default": "./lib/xterm.js"
  },
  "./css/xterm.css": "./css/xterm.css"
}

No source files are changed. All existing unit tests (2242) continue to pass.