SearchKit - Move grid css to its own file

[colemanw]

Overview

Moves css code for Grid search displays to a more appropriate location.

Before

Located in civicrm.css, which is not loaded by all themes.

After

Located in a css file specific to the searchDisplayGrid module.

[civibot]

(Standard links)

• If this is your first pull-request for CiviCRM, please browse CONTRIBUTING.md for information about the development and testing processes.
• If you are reviewing this pull-request, you may wish to consult the test sites and the Review Standards (long template, short template).

[AHowiller]

(CiviCRM Review Template WORD-1.2)

• General standards
  • (r-explain) Pass
  • (r-user) Pass: no impact
  • (r-doc) Pass
  • (r-run) Pass: runs fine
• Developer standards
  • (r-tech) Pass: improves file structure
  • (r-code) Pass: was just moved
  • (r-maint) Pass
  • (r-test) Pass: not needed

[colemanw]

Thanks @AHowiller