fix: handle git installation of ffmpeg

[SethFalco]

Description

Resolves the error that occurred when using an unconventional distribution of FFmpeg with a non-standard version string. For example, compiled from source, or a third-party fork.

• #getFFmpegVersion returns the full version string of the FFmpeg now, regardless of format.
• #parseSemVersion returns null if the string is not a semantic version string.
• #parseSemVersion handles normalization, such as if the patch segment is missing.
• If the FFmpeg version does not follow semver (so #parseSemVersion returns null), then we log a warning that we can't check compatibility.

Related issues

• Fixes Cannot check ffmpeg version #5911

Has this been tested?

• 👍 yes, I added tests to the test suite
• 💭 no, because this PR is a draft and still needs work
• 🙅 no, because this PR does not update server code
• 🙋 no, because I need help

[SethFalco]

Just a heads-up, I don't think this check was ever working properly.
The promise here wasn't being resolved, so the caller was probably just awaiting forever when the version was parsed successfully.

It just looked like everything was running smoothly, since the #checkFFmpegVersion method was a non-blocking call.

[SethFalco]

I actually compiled and installed n6.1-dev, which showed this version string.
Not sure if that's a typo or what, but a bit odd that the tag and version name don't match up. 🤔

[SethFalco]

Hmm, seems to me the failing test isn't related to this PR.
Even locally, this test fails on a build from a clean develop branch.

From my local machine:

mocha -c --timeout 30000 --exit --bail ./dist/server/tests/api/search/search-index.js

  Test index search
    Default search
      ✔ Should make a local videos search by default (138ms)
      ✔ Should make a local channels search by default
      ✔ Should make an index videos search by default (363ms)
      ✔ Should make an index channels search by default (165ms)
    Videos search
      ✔ Should make a simple search and not have results (420ms)
      ✔ Should make a simple search and have results (243ms)
      1) Should make a simple search


  6 passing (7s)
  1 failing

  1) Test index search
       Videos search
         Should make a simple search:

      AssertionError: expected 2 to equal 1
      + expected - actual

      -2
      +1
      
      at /home/seth/Documents/repos/contributor/Chocobozzz/PeerTube/dist/server/tests/api/search/search-index.js:82:55
      at Generator.next (<anonymous>)
      at fulfilled (node_modules/tslib/tslib.js:164:62)
      at process.processTicksAndRejections (node:internal/process/task_queues:95:5)

[Chocobozzz]

Thanks!