shell: new package for representing shell init configs internally

New types for the internal representation of the shell init configs,
which currently consists of files and key-value pairs for environment
global variables.

internal/shell/shell.go

@@ -0,0 +1,11 @@
+package shell
+
+type EnvironmentVariable struct {
+	Key   string
+	Value string
+}
+
+type InitFile struct {
+	Filename  string
+	Variables []EnvironmentVariable
+}