misc: Added SDL_EncodeURL.

[icculus]

• I confirm that I am the author of this code and release it to the SDL project under the zlib license. This contribution does not contain code from other sources, including code generated by a Large Language Model ("AI").

Adds a way to encode URL strings, for use with SDL_OpenURL(), etc. Generally you would encode either just the params and append them to a URL ("https://example.com?my_param=" + SDL_EncodeURL(SomeUntrustedString, NULL)), or the whole string for a file:/// path (file:/// + SDL_EncodeURL(SomeUntrustedPath, "\\/")).

Can be seen in action with:

./test/testurl file:///Some Path/That Has Spaces --urlencode --no-encode-chars ':/'

(Although strictly speaking, you would want to encode the path with just '/' in no-encode-chars and prepend "file://" directly, but this is just a test program.)

Fixes #16254.