Fixes the following compilation error found while dogfooding Builder
before release.

application.vala:22.24-22.27: error: This access invalid outside of instance methods

The goal here is to ensure we are less likely to spawn language servers
unless there is actually a need for the language server. For now, most
of these can be determined by the plugin type. However for search providers
they are enabled when the user types into the search box and begins typing.

The last thing we want to do while starting interactive search is:

 1) spawn new processes that can take away interactivity
 2) spawn a language server that has nothing to do with the project

I don't want this LSP which has not been updated to use IdeLspClient
enabled by default in builds.

Only load this completion provider for languages where it is useful.

ide_completion_context_get_bounds does not include `$` in the bounds so
when we delete (begin, end) from the text buffer so we can insert the
text provided by the completion we don't remove the `$`. For php this is
problematic because the completion includes the `$` prefix, so when we
insert the completion we end up with 2 `$`.

For example `$t` triggers autocomplete with a proposal `$tree`, we apply the
proposal and end up with `$$tree`.

This patch also removes the common prefix of the completion item and the
`begin` GtkTextIter.

Fixes #1641