Allow lts for the LTS, and allow pre for the latest pre-release (#234)

* update tests to latest `versions.json`

* allow specifying the version as `lts` to install the latest LTS version

for now, this just hardcodes the LTS version in the source (similar to how juliaup does it) since the latest LTS is not available in `versions.json`. Since the LTS is updated so rarely this might be ok for now.

* add a 'pre' version

* Run `npm ci` followed by `npm run build`

* Add a CI job to test `pre`

---------

Co-authored-by: Viral B. Shah <ViralBShah@users.noreply.github.com>
Co-authored-by: Dilum Aluthge <dilum@aluthge.com>

lib/installer.js

@@ -42,6 +42,8 @@ const os = __importStar(require("os"));
 const path = __importStar(require("path"));
 const retry = require("async-retry");
 const semver = __importStar(require("semver"));
+const LTS_VERSION = '1.6';
+const MAJOR_VERSION = '1'; // Could be deduced from versions.json
 // Translations between actions input and Julia arch names
 const osMap = {
     'win32': 'winnt',
@@ -111,6 +113,12 @@ function getJuliaVersion(availableReleases, versionInput, includePrerelease = fa
         // versionInput is a valid version or a nightly version, use it directly
         return versionInput;
     }
+    if (versionInput == 'lts') {
+        return getJuliaVersion(availableReleases, LTS_VERSION, false);
+    }
+    if (versionInput == 'pre') {
+        return getJuliaVersion(availableReleases, MAJOR_VERSION, true);
+    }
     // Use the highest available version that matches versionInput
     let version = semver.maxSatisfying(availableReleases, versionInput, { includePrerelease });
     if (version == null) {