Improve nested variables support

Closes #76 #89 #77 #83

testdata/vars/v2/multiline/Taskfile.yml

@@ -0,0 +1,45 @@
+version: 2
+tasks:
+  default:
+    vars:
+      MULTILINE: "\n\nfoo\n  bar\nfoobar\n\nbaz\n\n"
+    cmds:
+      - task: file
+        vars:
+          CONTENT:
+            sh: "echo 'foo\nbar'"
+          FILE: "echo_foobar.txt"
+      - task: file
+        vars:
+          CONTENT:
+            sh: "echo -n 'foo\nbar'"
+          FILE: "echo_n_foobar.txt"
+      - task: file
+        vars:
+          CONTENT:
+            sh: echo -n "{{.MULTILINE}}"
+          FILE: "echo_n_multiline.txt"
+      - task: file
+        vars:
+          CONTENT: "{{.MULTILINE}}"
+          FILE: "var_multiline.txt"
+      - task: file
+        vars:
+          CONTENT: "{{.MULTILINE | catLines}}"
+          FILE: "var_catlines.txt"
+      - task: enumfile
+        vars:
+          LINES: "{{.MULTILINE}}"
+          FILE: "var_enumfile.txt"
+  file:
+    cmds:
+      - |
+        cat << EOF > '{{.FILE}}'
+        {{.CONTENT}}
+        EOF
+  enumfile:
+    cmds:
+      - |
+        cat << EOF > '{{.FILE}}'
+        {{range $i, $line := .LINES| splitLines}}{{$i}}:{{$line}}
+        {{end}}EOF