fix: Make numeric names valid

[aleksanderkatan]

This may actually happen when using externals, since we name unnamed items based on the external key.

    const myBuf = root.createMutable(d.u32).$name('0');
    const fn = () => {
      'use gpu';
      myBuf.$ = 2;
    };

    expect(tgpu.resolve([fn])).toMatchInlineSnapshot(`
      "@group(0) @binding(0) var<storage, read_write> 0: u32;

      fn fn_1() {
        0 = 2u;
      }"
    `);