feat: support -g, --global install arg

[marvinhagemeister]

This PR adds support for installing packages globally via jsr i -g <package>. This allows jsr to self update via jsr i -g jsr for example.

Not sure how to write tests for this as the other package managers don't seem to have a way to temporarily overwrite the global install location.

Fixes #46

[ayushkavad]

@marvinhagemeister I've conducted a series of test cases to validate the global installation feature introduced in this PR:

describe('global install', () => {
  it('jsr i global --npm @std/encoding', async () => {
    await withTempEnv(
      ['i', 'global', '--npm', '@std/encoding'],
      async (dir) => {
        const nodeModules = path.join(dir, 'node_modules', '@std', 'encoding');
        assert.ok(
          await isDirectory(nodeMoules),
          ' node_modules path does not exist',
        );
      },
    );
  });

  it('jsr i global --yarn @std/encoding', async () => {
    await withTempEnv(
      ['i', 'global', '--yarn', '@std/encoding'],
      async (dir) => {
        const nodeModules = path.join(dir, 'node_modules', '@std', 'encoding');
        assert.ok(
          await isDirectory(nodeMoules),
          ' node_modules path does not exist',
        );
      },
    );
  });

  it('jsr i global --pnpm @std/encoding', async () => {
    await withTempEnv(
      ['i', 'global', '--pnpm', '@std/encoding'],
      async (dir) => {
        const nodeModules = path.join(dir, 'node_modules', '@std', 'encoding');
        assert.ok(
          await isDirectory(nodeMoules),
          ' node_modules path does not exist',
        );
      },
    );
  });

  it('jsr i global --bun @std/encoding', async () => {
    await withTempEnv(
      ['i', 'global', '--bun', '@std/encoding'],
      async (dir) => {
        const nodeModules = path.join(dir, 'node_modules', '@std', 'encoding');
        assert.ok(
          await isDirectory(nodeMoules),
          ' node_modules path does not exist',
        );
      },
    );
  });
});