feat mr: remove prefix in ID of MR just like Issue

Created by: atusy

This change

• improves consistency of APIs.
• is copied from: https://github.com/profclems/glab/blob/0232a0c0791374ef5c1fa2072582a7127126b078/commands/issue/issueutils/utils.go#L106
• makes pipes easier
  • e.g., glab mr list | fzf | grep -oE "^\S+" | glab mr view
  • this is what atusy/gh-fzf internally does

Description

This change adds support for a prefix in MR ID like below.

glab mr view \!1

Related Issue

Resolves #789 (closed)

How Has This Been Tested?

Run below on manjaro linux.

glab repo clone gitlab-org/gitlab-foss -- --depth 1
glab mr view "\!1"

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)