Add basic Lua ALE functions and test coverage

Ensure that basic ALE functions `ale.var`, `ale.escape`, and `ale.env`
are available in Lua. Cover all Lua code so far with busted tests,
fixing bugs where ALE variables can be set with Boolean values instead
of numbers. Document all functionality so far.

Dockerfile

@@ -2,12 +2,10 @@ ARG TESTBED_VIM_VERSION=24
 
 FROM testbed/vim:${TESTBED_VIM_VERSION}
 
-RUN install_vim -tag v8.0.0027 -build \
-                -tag v9.0.0297 -build \
-                -tag neovim:v0.7.0 -build \
-                -tag neovim:v0.8.0 -build
-
 ENV PACKAGES="\
+    lua5.1 \
+    lua5.1-dev \
+    lua5.1-busted \
     bash \
     git \
     python2 \
@@ -19,6 +17,11 @@ ENV PACKAGES="\
 RUN apk --update add $PACKAGES && \
     rm -rf /var/cache/apk/* /tmp/* /var/tmp/*
 
+RUN install_vim -tag v8.0.0027 -build \
+                -tag v9.0.0297 -build \
+                -tag neovim:v0.7.0 -build \
+                -tag neovim:v0.8.0 -build
+
 RUN pip install vim-vint==0.3.21
 
 RUN git clone https://github.com/junegunn/vader.vim vader && \