feat: add SELinux policy source files

[dschapman]

This PR adds support for the SELinux policy source file type. This type is different from the CIL file type already present in tokei. The three extensions this PR adds (.te, .fc, .if) all have the same comment syntax.

A Redhat article about SELinux policy defines the three files as follows:

The generated policy template contains the following policy files:

.te - defines policy rules as well as new types and domains used by your application
.fc - contains file context definitions, in other words, instructions for labeling files related to the application
.if - contains interfaces, which are policy macros used by other policy modules to interact with this policy