Validate that SSH key file path is not a directory

[isc-pbarton]

Description

Resolves #943

Adds validation to confirm the SSH key file path is not a directory when configuring Embedded Git, both through the Configure() prompt and the Settings page.

Testing

In the development container:

• do ##class(SourceControl.Git.API).Configure() to go through the configuration prompts. When prompted for ssh key file path, put in /usr/irissys/mgr: the Configure prompt displays an informative error and quits out at the end. Try again with /usr/irissys/mgr/test_key.pub: at the end it prompts you to create the SSH key, and successfully does so.
• Visit the settings page at http://localhost:52774/isc/studio/usertemplates/gitsourcecontrol/gitprojectsettings.csp and test directory and non-directory values in the "SSH Private Key File" input.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 49.10%. Comparing base (6438970) to head (6816e63).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #966   +/-   ##
=======================================
  Coverage   49.10%   49.10%           
=======================================
  Files          23       23           
  Lines        3287     3287           
=======================================
  Hits         1614     1614           
  Misses       1673     1673           

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[isc-dchui]

What's wrong with $$$ISOK()?