Make case insensitivity optional for generateSemiGenericRegex and generateUniqueTokenRegex

[rgmz]

Description:

This makes it possible to toggle whether generateSemiGenericRegex and generateUniqueTokenRegex inserts the case-insensitivity flag. There should be no functional or config changes introduced by this PR.

Context: #1204 (comment)

Checklist:

• Does your PR pass tests?
• Have you written new tests for your changes?
• Have you lint your code locally prior to submission?

[rgmz]

For generateSemiGenericRegex it might be useful to make the identifiers and secretRegex independently case-insensitive.

e.g.,
Pattern

# simplified pattern
(?i:snyk.+?)=(abc123)

Detections

# Match the identifier regardless of case
SNYK_TOKEN=abc123
snyk_token=abc123
# Don't match the secret if it's uppercase
SNYK_TOKEN=ABC123
snyk_token=ABC123

https://pkg.go.dev/regexp/syntax

[zricethezav]

@rgmz I agree with these changes -- they're simple enough and it's a drop in addition that might help folks tweak their rules a little better. At first I thought supporting this might be more work since we have a keyword check that is case insensitive since we're converting the values ToLower, but, after reviewing

gitleaks/detect/detect.go

Line 260 in 3cbcda2

matchIndices := rule.Regex.FindAllStringIndex(fragment.Raw, -1)

it looks like fragment.Raw is not converted ToLower.

If you can resolve the merge conflicts, I'm happy to merge this :)

[rgmz]

@zricethezav done.