Feature: Add support for named-pipe sockets for LSPs

[Svetlitski]

Overview:

Previously, ALE assumed that for all linters with lsp == 'socket' that the socket referred to by address was a TCP socket. Neovim's sockconnect also supports named pipes [1] which have lower overhead than TCP sockets. This pull-request thus adds support for using sockets which are named pipes. The very simple heuristic of checking if address contains a colon or not is used to determine if the given socket is TCP or a named pipe, in order to maintain backwards compatibility, and to avoid adding an additional configuration option to ale#linter#Define.

Tests:
A new test case has been added to test/test_socket_connections.vader in order to test named pipe sockets. To support this test, I wrote a very simple named-pipe-based server test/dumb_named_pipe_server.py (which is largely copied from test/dumb_tcp_server.py)

Documentation:

The documentation has been updated with information about this new feature.

^{[1] What I'm calling "named pipes" here are not actually named pipes (i.e. the things you'd get from calling `mkfifo`), but unix domain sockets. For whatever reason Neovim's documentation conflates these things (see `:help sockconnect`), so I've just tried to be consistent with their terminology}

[kevinclark]

LGTM! 👍

[hsanson]

ALE migrated from Travis to Github Actions (see #3548) so it is necessary to rebase this PR from latest master branch to trigger the required checks.

# Add dense-analysis as remote upstream on your local fork. Not 
# needed if already added.
git remote add upstream https://github.com/dense-analysis/ale.git

# Sync your local master with upstream master
git checkout master
git fetch upstream master
git merge upstream/master

# Rebase the PR branch with master
git checkout my-branch-name
git rebase master   # Fix any conflicts you may have

# Force push the updated PR branch to origin to trigger the checks
git push -f origin my-branch-name

[Svetlitski]

Closed by accident, whoops. @hsanson, done, looks like everything is passing.

[w0rp]

I'm happy with this. Pretty good addition.

[w0rp]

Cheers! 🍻