Fix CI rules-if comparison with Regexp variables

What does this MR do and why?

When the right side of a =~ or !~ comparison is a regexp variable, we consider this as a string. However, we need to try converting it to a regexp if possible.

In this MR, we start to try to consider the right side of an expression as a regexp.

Related to #35438 (closed)

This fix is behind a feature flag ci_fix_rules_if_comparison_with_regexp_variable (#359740 (closed)).

Screenshots or screen recordings

1. Example CI pipeline config:

variables:
  teststring: 'abcde'
  pattern: '/^ab.*/'

test1:
  script: exit 0
  rules:
    - if: '$teststring =~ $pattern'

test2:
  script: exit 0
  rules:
    - if: '$teststring =~ /^ab.*/'

2. Run a pipeline;

test1 is not included in the pipeline.

3. Enable ci_fix_rules_if_comparison_with_regexp_variable and run a pipeline;

Feature.enable(:ci_fix_rules_if_comparison_with_regexp_variable)

test1 is included in the pipeline.

MR acceptance checklist

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lib/gitlab/ci/pipeline/expression/lexeme/matches.rb

@@ -11,8 +11,15 @@ class Matches < Lexeme::LogicalOperator
             def evaluate(variables = {})
               text = @left.evaluate(variables)
               regexp = @right.evaluate(variables)
+
               return false unless regexp
 
+              if ::Feature.enabled?(:ci_fix_rules_if_comparison_with_regexp_variable)
+                # All variables are evaluated as strings, even if they are regexp strings.
+                # So, we need to convert them to regexp objects.
+                regexp = Lexeme::Pattern.build_and_evaluate(regexp, variables)
+              end
+
               regexp.scan(text.to_s).present?
             end