ENH: Add input validation for app name

[jamesobutler]

This closes #102.

Per https://cookiecutter.readthedocs.io/en/stable/advanced/hooks.html#examples:

A pre_gen_project hook can validate template variables.

I have used this method to return an exit failure if an app_name is provided with anything other than alphanumeric characters.

  [1/12] project_name (SlicerCustomAppTemplate): 3D Slicer
  [2/12] github_organization (Kitware):
  [3/12] github_project (3D Slicer):
  [4/12] org_domain (kitware.com):
  [5/12] org_name (Kitware, Inc.):
  [6/12] app_name (3D Slicer):
  [7/12] bundle_identifier (com.kitware.3d slicer):
  [8/12] app_version_major (0):
  [9/12] app_version_minor (1):
  [10/12] app_version_patch (0):
  [11/12] app_description_summary (Customized version of Slicer):
  [12/12] cdash_drop_site (open.cdash.org):
ERROR: 3D Slicer is not a valid app name. Only alphanumeric characters are allowed.
ERROR: Stopping generation because pre_gen_project hook script didn't exit successfully
Hook script failed (exit status: 1)

[jcfr]

🙏 for the patch. This is also a great improvement 💯

Could you check that the following also work:

• Can't generate project whose project/app name starts with a lowercase letter #19

[jamesobutler]

Using the same type inputs as in the original issue report of #19, I'm unable to replicate on Windows when using this branch. I haven't tried with current main,

PS C:\D> cookiecutter gh:jamesobutler/SlicerCustomAppTemplate -c app-name-validation
You've downloaded C:\Users\butlej30383\.cookiecutters\SlicerCustomAppTemplate before. Is it okay to delete and
re-download it? [y/n] (y): Y
  [1/12] project_name (SlicerCustomAppTemplate): aevaSlicer
  [2/12] github_organization (Kitware):
  [3/12] github_project (aevaSlicer): aevaSlicer
  [4/12] org_domain (kitware.com):
  [5/12] org_name (Kitware, Inc.):
  [6/12] app_name (aevaSlicer): aevaSlicer
  [7/12] bundle_identifier (com.kitware.aevaslicer):
  [8/12] app_version_major (0):
  [9/12] app_version_minor (1):
  [10/12] app_version_patch (0):
  [11/12] app_description_summary (Customized version of Slicer):
  [12/12] cdash_drop_site (open.cdash.org):
PS C:\D> python --version
Python 3.12.10
PS C:\D> cookiecutter --version
Cookiecutter 2.6.0 from C:\Users\butlej30383\AppData\Local\Programs\Python\Python312\Lib\site-packages (Python 3.12.10 (tags/v3.12.10:0cc8128, Apr  8 2025, 12:21:36) [MSC v.1943 64 bit (AMD64)])
PS C:\D>