LibGfx/TGA: Refactor the plugin interface

[LucasChollet]

~~Depends on #19894.~~ Merged! Contributes to #19893

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LibGfx/TGA: Default initialize the TGAHeader in the context

LibGfx/TGA: Make TGAImageDecoderPlugin::decode_tga_header() fallible

LibGfx/TGA: Decode the header in create() and remove initialize()

This is done as a part of #19893.

file: Register the "image/x-targa" mime-type

[AtkinsSJ]

Conflict time! ✨