feat(css): Add new width variable

What does this MR do?

feat: adding new width variable

• in several locations in our app we are using w-15p, so I added gl-w-15p

❯ git grep w-15p
app/assets/javascripts/alert_management/components/alert_management_table.vue:      thClass: `${thClass} js-started-at w-15p`,
app/assets/javascripts/alert_management/components/alert_management_table.vue:      thClass: `${thClass} w-15p`,
app/assets/javascripts/error_tracking/components/error_tracking_list.vue:      thClass: 'w-15p',
app/assets/javascripts/incidents/components/incidents_list.vue:      thClass: `${thClass} w-15p`,
app/assets/javascripts/incidents/components/incidents_list.vue:      thClass: 'gl-pointer-events-none w-15p',
app/assets/javascripts/incidents/components/incidents_list.vue:      thClass: `${thClass} w-15p`,
app/assets/stylesheets/framework/common.scss:.w-15p { width: 15%; }

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• Added the ~"component:*" label(s) if applicable.

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