Add support for Jenkinsfile syntax (#2750)

Build pipelines for the Jenkins build system are configured in Groovy,
however since their filename is always `Jenkinsfile`, micro doesn't
recognize them as Groovy, and doesn't add syntax highlighting.

This small commit simply adds `Jenkinsfile` and `jenkinsfile` as file
names recognized as Groovy.

runtime/syntax/groovy.yaml

@@ -1,7 +1,7 @@
 filetype: groovy
 
 detect:
-    filename: "\\.(groovy|gy|gvy|gsh|gradle)$"
+    filename: "(\\.(groovy|gy|gvy|gsh|gradle)$|^[Jj]enkinsfile$)"
     header: "^#!.*/(env +)?groovy *$"
 
 rules: