Suggestion:  Further File Extensions
At the moment, the following languages are supported:

- Nim
- Bash
- C
- C++
- C#
- Java
- JavaScript
- Markdown
- Python
- YAML

For some of these languages, there are some further common file extensions which are either supported by the official IDEs, important compilers or common documentation systems.  In the following, those further file extensions are listed together with a brief explanation as well as their references.

### C

C source files without preprocessing; GCC manual page for v10.2.1:

- `*.i`

Doxygen's documentation container format (a subset of C; pure C-style comment file); Doxygen v1.9.1:

- `*.dox`

### C++

C++ source files without preprocessing; GCC manual page v10.2.1:

- `*.ii`

C++ source files with preprocessing; GCC manual page for v10.2.1:

- `*.C`
- `*.cp`
- `*.c++`
- `*.cxx`
- `*.CPP`

C++ headers; GCC manual page for v10.2.1:

- `*.H`
- `*.hh`
- `*.hp`
- `*.h++`
- `*.hxx`
- `*.HPP`
- `*.tcc`

### JavaScript

TypeScript language (an extension to JavaScript):

- `*.ts`

### Markdown

alternative extension; Doxygen v1.9.1:

- `*.markdown`

### Python

alternative extension; IDLE v3.9.2 (official Python 3 IDE):

- `*.pyw`

Cython language (an extension to Python; C interface specification language):

- `*.pyx`

### YAML

[Citation File Format](https://github.com/citation-file-format/citation-file-format) (a subset of YAML; GitHub's software citation standard):

- `*.cff`

## Conclusion

As the GNU Compiler Suite is the most important compiler collection for established ahead-of-time compiled languages, the extensions it supports are quite likely to appear in the wild.  These extensions are most important, in my opinion.

Doxygen is a common documentation utility for C-like languages and some others, including Python, VHDL, PHP and Fortran.  Especially when developing Fortran 2018 projects, there is no real alternative to Doxygen.  It supports some alternative extensions in addition to other systems.  These extensions are rather optional but should be discussed for the next major release.

Language extensions are usually a (real) superset of the respective language such that there are some features which do not exist in the base language.  They have their applications and audiences; Cython is mostly applied in numerical sciences in order to speed up `numpy` calculations while TypeScript became the quasi-standard for all purposes which demand JavaScript, including GitHub Actions.  Even if only the base language is supported with syntax highlighting, any visual support helps during the development process.  It should be discussed how these languages could be supported.

Last but not least, there are the (real) subsets.

The Doxygen container format has its own rules within a C-like comment.  If supported at all, it should, hence, be considered C-like.  Doxygen has its own command set, thus, a dedicated support might be interesting in the long term.  It is quite exotic and requiring a Doxygen container often, at least in my opinion, indicates issues regarding the project's design.  I added it to the list for completeness.

CFF is something I frequently offer to set it up for a repository; if you are interested, @fox0430, I can do so for this repository, as well.  If configured, GitHub will render a "Cite this repository" blob with citation meta data for the respective repository to be copied and pasted to a list of references in either a publication and / or a software documentation.  This can be considered free advertisement for the respective repository.  It is a subset of YAML to be matched against a JSON schema for validity.  CFF is mostly compatible to BibTeX, a conversion between them is possible -- GitHub does so with the "Cite this repository" blob.  In order to distinct it from other configuration files, the CFF maintainers decided to give a unique and plausible extension, `*.cff`.  If supported, Moe would become the first editor which supports CFF out-of-the-box.  There is a catalogue of CFF tools in the main repository listing all known utilities.  If Moe would support the format with highlighting, adding it to that catalogue could be requested, enhancing the visibility of this project.  I added this format to the above list for completeness.