[gp] 'gp preview <url>' should work in Code and other IDEs

[jankeromnes]

• Depends on [code] support embedded live preview #2806
• Similar to gp cli: open and preview support for other IDEs #3309, make gp preview <url> try:
  a. Theia
  b. GP_PREVIEW_BROWSER
  c. fail

[jankeromnes]

(Added to March milestone as well because issue was split off from #3309)