Move `contextive.path` default to `Contextive.LanguageServer` for vscode extension about contextive

[chrissimon-au]

Noting that the config value defined in the vscode extension is currently used by the initialise command to know where to deposit the default file. Suggest using this as an opportunity to move the initialize handling to a custom lsp command handled in the language server, so it can be leveraged by other implementations as well.

[chrissimon-au]

We have switched to using the language server custom contextive/initialize command now, but kept the default value in the contextive vscode extension package.json for two reasons:

1. keeping the default value allows vscode to show the default value in the settings editor, which is clearer UX
2. when the default value is null, the omnisharp library fails to retrieve the settings

Since we now have the default contextive.path value defined in two places (vscode extension and language server), the e2e test of the vscode extension acts as a guard against the default value changing, and the error message marks that it should match the default defined in the server.