[BUILD-960] fix: Tag-to-slug conversion with number later in tag part

[RisingOrange]

For some tags the tag-to-slug conversion results in a slug with extra numbers.

For example:
#AK_Step1_v12::#FirstAid::01_Biochem::03_Laboratory_Techniques::02_CRISPR/Cas9
currently results in this slug:
step1-fa-1-3-29
while it should be this:
step1-fa-1-3-2

Related issues

https://ankihub.atlassian.net/jira/software/c/projects/BUILD/boards/1?selectedIssue=BUILD-960

Proposed changes

• Only use numbers from beginning of the tag parts
• Handle invalid tags where tag part doesn't start with number correctly - by returning None
• Add tests
• Update pycln (precommit)
  • This fixes an error that was happening in the precommit GHA