The most common use of the text editor seems to be for scripting. Having
line numbers and syntax highlighting enabled by default seems sensible.
Syntax highlighting is now enabled by default, but is automatically
disabled when the datablock has a non-highlighted extension.
Highlighting is enabled for filenames like:
    - Text
    - Text.001
    - somefile.py
and is automatically disabled when the datablock has an extension for
which Blender has no syntax highlighter registered.
Reviewers: billreynish, campbellbarton
Subscribers: brecht, billreynish
Differential Revision: https://developer.blender.org/D5472