Currently, we detect the linter root based on a variety of techniques.
However, these techniques are not foolproof. For example, clangd works
fine for many things without a compile_commands.json file, and Go
projects may be built outside of the GOPATH to take advantage of Go
1.11's automatic module support.
Add global and buffer-specific variables to allow the user to specify
the root, either as a string or a funcref. Make the funcrefs accept the
buffer number as an argument to make sure that they can function easily
in an asynchronous environment.
We define the global variable in the main plugin, since the LSP linter
code is not loaded unless required, and we want the variable to be able
to be read correctly by :ALEInfo regardless.
This is the callback-based variant of the existing `lsp_config` linter
option. It serves the same purpose but can be used when more complicated
processing is needed.
`lsp_config` and `lsp_config_callback` are mutually exclusive options;
if both an given, a linter preprocessing error will be raised.
The runtime logic has been wrapped in `ale#lsp_linter#GetConfig` for
convenience, similar to `ale#lsp_linter#GetOptions`.
This also adds documentation and an `AssertLSPConfig` test function for
completeness.
