feat: case_sensitive option for highlight and sanitize filter

[shouya]

This PR adds a case_sensitive option to the highlight and sanitize filters. By default, it is set to false.

When case_sensitive is true for the highlight filter, it will only highlight matches that have the same case as the search term.

Similarly, case_sensitive controls whether the from field of the replace operation in the sanitize filter matches the case. Note that currently, it's not possible to specify a case-sensitive remove operation. As a workaround, you
can use a case-sensitive replace operation with to: "".

Example:

highlight:
  keywords:
    - foo
    - bar
  bg_color: '#ffff00'
  case_sensitive: true

sanitize:
  - remove: "foo"
  - remove_regex: '\\d+'
  - replace:
      from: "bar"
      to: "baz"
      case_sensitive: true
  - replace_regex:
      from: '\\w+'
      to: "qux"
      case_sensitive: true