Rename "develop" option to "editable"

We should rename the dependency option develop to editable to align with what it actually is and to avoid confusion with a
"development dependency".

- foo = { path = "./path/to/foo", develop = true }
+ foo = { path = "./path/to/foo", editable = true }

Additionally we should also introduce this as a commandline option to poetry add (eg: poetry add --editable x).

Changes Required

• Update specification schema in poetry-core.
• Update dependency instances to reference editable and fallback to develop if not present.
• Update references to Dependency.develop in both poetry-core and poetry.
• Add deprecation warning when accessing Dependency.develop.
• Add poetry check warning to inform users about the change.
• Add --editable option directory and vcs dependencies to poetry add command. (This can be an issue on its own.)