Add CI scripts to install and link JS SDK

This installs and links the JS SDK to access ESLint rules and their deps. These
scripts are copied from the React SDK.

Part of https://github.com/vector-im/riot-web/issues/13584

scripts/ci/install-deps.sh

@@ -0,0 +1,13 @@
+#!/bin/bash
+
+set -ex
+
+scripts/fetchdep.sh matrix-org matrix-js-sdk
+
+pushd matrix-js-sdk
+yarn link
+yarn install $@
+popd
+
+yarn link matrix-js-sdk
+yarn install $@

scripts/fetchdep.sh

@@ -0,0 +1,40 @@
+#!/bin/bash
+
+set -x
+
+deforg="$1"
+defrepo="$2"
+defbranch="$3"
+
+[ -z "$defbranch" ] && defbranch="develop"
+
+rm -r "$defrepo" || true
+
+clone() {
+    org=$1
+    repo=$2
+    branch=$3
+    if [ -n "$branch" ]
+    then
+        echo "Trying to use $org/$repo#$branch"
+        git clone git://github.com/$org/$repo.git $repo --branch "$branch" --depth 1 && exit 0
+    fi
+}
+
+# Try the PR author's branch in case it exists on the deps as well.
+# If BUILDKITE_BRANCH is set, it will contain either:
+#   * "branch" when the author's branch and target branch are in the same repo
+#   * "author:branch" when the author's branch is in their fork
+# We can split on `:` into an array to check.
+BUILDKITE_BRANCH_ARRAY=(${BUILDKITE_BRANCH//:/ })
+if [[ "${#BUILDKITE_BRANCH_ARRAY[@]}" == "1" ]]; then
+    clone $deforg $defrepo $BUILDKITE_BRANCH
+elif [[ "${#BUILDKITE_BRANCH_ARRAY[@]}" == "2" ]]; then
+    clone ${BUILDKITE_BRANCH_ARRAY[0]} $defrepo ${BUILDKITE_BRANCH_ARRAY[1]}
+fi
+# Try the target branch of the push or PR.
+clone $deforg $defrepo $BUILDKITE_PULL_REQUEST_BASE_BRANCH
+# Try the current branch from Jenkins.
+clone $deforg $defrepo `"echo $GIT_BRANCH" | sed -e 's/^origin\///'`
+# Use the default branch as the last resort.
+clone $deforg $defrepo $defbranch