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>

src/installer.ts

@@ -10,6 +10,9 @@ import retry = require('async-retry')
 
 import * as semver from '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',
@@ -82,6 +85,14 @@ export function getJuliaVersion(availableReleases: string[], versionInput: strin
         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) {

src/setup-julia.ts

@@ -1,5 +1,4 @@
 import * as core from '@actions/core'
-import * as exec from '@actions/exec'
 import * as tc from '@actions/tool-cache'
 
 import * as fs from 'fs'