[TODO] [pending popularity] add extension nimf for Nim

[timotheecour]

/cc @pchaigno

Description

• hopefully the last followup on add extensions for nim files #4295
• adds nimf extension for Nim language

Note 1: the nimf extension (for source code filter Nim files) was previously called tmpl but that was causing confusion because the extension tmpl was too generic and also didn't match the other Nim extensions (nim, nims, nimble, nim.cfg), so nim-lang/Nim#9658 was recently merged to change the tmpl extension to nimf (while retaining backward compatibility); note that currently nimf is only used here: https://github.com/search?q=extension%3Animf+stdtmpl&type=Code ; but the old extension tmpl (which fits the same purpose as nimf) is used in more places, see https://github.com/search?q=extension%3Atmpl+stdtmpl&type=Code ; the plan is to gradually migrate the files using tmpl extension to nimf, and having it supported in linguist will provide extra incentive for the migration (eg syntax highlight), although linguist integration wasn't the only reason for the tmpl=>nimf change.

Note 2: nimf doesn't look like it's being used for anything but Nim source code filters

Checklist:

• I am associating a language with a new file extension.
  • The new extension is used in hundreds of repositories on GitHub.com (but, see note above)
    • Search results for each extension:
      https://github.com/search?q=extension%3Animf+stdtmpl&type=Code
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • https://github.com/nim-lang/Nim/blob/devel/tools/niminst/makefile.nimf
    • Sample license(s): https://github.com/nim-lang/Nim/blob/devel/copying.txt
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

[pchaigno]

Usage is way too low to add support in Linguist for now. Alternatively, if you think usage will grow to meet our requirement of hundreds of repositories, I can add the extension to #4219. Note that currently, even the usage for .tmpl is too low.