Add a marker of the target language of databases in the tree view

[aeisenberg]

This is a subtask of #318.

[aeisenberg]

@adityasharad How can I get access to the target language? I see in all of my database directories there is a refresh file that contains something like db-javascript or db-cpp, but I don't know if that's stable enough to trust.

[adityasharad]

Good question. There is sadly no fully principled way of doing this, because in theory the CLI and database do not know which language the database is for.

I suggest one of two approaches:

• Look at the name of the db-<lang> dataset directory within the database (this is db.contents.datasetUri), and maintain a mapping from <lang> slugs to the human-readable language names. (The <lang> slugs are consistent with the folder names in https://github.com/github/codeql, e.g. C/C++ is always cpp.)
• Look at the name of the DB scheme file within the database (this is db.contents.dbSchemeUri), and maintain a mapping from DB scheme file names to the human-readable language names. (The DB scheme file names are always semmlecode.<lang>.dbscheme, except for Java which is just semmlecode.dbscheme.)