Prepare code for Elixir 1.18

[rNoz]

As mentioned in PR opened by @camelpunch, this PR solves that issue while keeping the same spec as the original intention provided by the code base.

I have been reviewing the specification and other libraries (go, zig and javascript), and keeping the 0x2F (magic byte) is the best way to go, than removing it (approach taken by the mentioned PR).

Then, I have added Elixir 1.18 to the CI Test and Lint jobs (version matrix) and added --warnings-as-errors in mix compile.

Correcting badges (README) and incorporating 3 new github action workflows as a replacement for Travis.

Finally, bumping versions of all dependencies.

58 Tests and 91.5% Code Coverage

.........................................................
Finished in 0.1 seconds (0.1s async, 0.00s sync)
6 doctests, 52 tests, 0 failures
----------------
COV    FILE                                        LINES RELEVANT   MISSED
 93.8% lib/ex_image_info.ex                          341       49        3
  0.0% lib/ex_image_info/detector.ex                  10        0        0
100.0% lib/ex_image_info/types/bmp.ex                 26        6        0
100.0% lib/ex_image_info/types/gif.ex                 34        9        0
 88.8% lib/ex_image_info/types/ico.ex                 53       18        2
100.0% lib/ex_image_info/types/jp2.ex                 29        6        0
 78.9% lib/ex_image_info/types/jpeg.ex                63       19        4
100.0% lib/ex_image_info/types/png.ex                 29        6        0
 95.8% lib/ex_image_info/types/pnm.ex                 94       24        1
100.0% lib/ex_image_info/types/psd.ex                 26        6        0
 89.1% lib/ex_image_info/types/tiff.ex               123       37        4
 85.7% lib/ex_image_info/types/webp.ex                90       21        3
[TOTAL]  91.5%

[warmwaffles]

Hah, I was looking at resolving the type errors yesterday. Got sucked into another issue on a different library.

[warmwaffles]

Side note, is committing the generated docs useful anymore? 8 years ago it made sense, but now hexdocs is not going anywhere and regenerating them is fairly simple.

[camelpunch]

Looks good. I haven't pored over every detail in this change, but it passes the tests in my PDF generator that uses this as a dependency and eliminates the warnings.

[camelpunch]

See above!

[rNoz]

I wasn't sure about this bump, but I considered also this comment and the CI changes of this PR.

[rNoz]

Not really required these 2 version updates, as mix.lock is committed, but well, since all is for dev/test, updating it here as well.

[rNoz]

No repo updates in last 3 years, CI service is down (although badges work), has some warnings when compiling. I would consider if removing this along this year.

[rNoz]

Did a bunch of changes, pinging again, and as any of you confirm it, I'll merge to have a new 0.2.7 version. Then, I'll rename master branch as main.

[rNoz]

Moreover, fixing all the badges for ex_doc, as it does not have raw html support in markdown (while GitHub does).

mix docs:

GitHub's rendered html.

[rNoz]

Decided to remove this file as that service was ok for open source projects back then, but I have just quickly checked and it seems they only offer priced plans.