Improve shared sensitive data library handling of snake_case variable names

[geoffw0]

Improve the share sensitive data library handling of snake_case variable names. This is especially relevant for Rust, where snake case is encouraged and widely used, but the main change is in the shared SensitiveDataHeuristics.qll file and will affect (hopefully benefit) other languages as well. It's a simple change, but we need a fairly high level of confidence / agreement before merging this.

Testing:

• CI / tests passing (if you add up the sensitive data tests across all languages, coverage is really quite good)
• review differences in sensitive data identified
  • Rust, all DCA projects (by running rust/summary/sensitive-data) --- experiment, 389 new sensitive expressions, LGTM
  • Swift, MRVA top-100 projects (by running swift/summary/sensitive-expressions) --- no differences
• review differences in query results (DCA)
  • Javascript
  • Python
  • Ruby
  • Rust
  • Swift
• check performance
  • swift/summary/sensitive-expressions (locally) --- no significant differences
  • swift/cleartext-logging (locally) --- no significant differences
  • DCA analysis times, all languages

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