feat(GlChartLegend): split into two components

What does this MR do?

Use legend.vue as a wrapper for two components:

<gl-inline-legend v-if="layout === 'inline'" />
<gl-tabular-legend v-if="layout === 'tabular'" />

Reference suggestion: !1327 (comment 338346854)

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • Customers Portal: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.