Improving error message for invalid file types

[PepinhoJp]

Closes #2486

Refactored code for loading main source file with utf-8 error detection, providing a helpful message. See examples below:

> ./typst compile typst

error: file is not valid utf-8
 = hint: a file without an extension (`.something`) is not usually a Typst file. Check if you meant to add `.typ` to its name.

> ./typst.exe compile typst.exe

error: file is not valid utf-8
 = hint: a file with the `.exe` extension is not usually a Typst file. Check if you meant to use `.typ` instead.

[MDLC01]

I'm not convinced the second part of the hint ("Check if you meant to add .typ to its name." / "Check if you meant to use .typ instead.") is useful. Changing the file's extension won't magically make its content valid UTF-8. Otherwise, I really like this hint!

[PgBiel]

I'm not convinced the second part of the hint ("Check if you meant to add .typ to its name." / "Check if you meant to use .typ instead.") is useful. Changing the file's extension won't magically make its content valid UTF-8.

I believe the rationale for this is in the original issue, where you may accidentally (for example) use tab autocomplete towards a previously generated PDF file with the same name instead of the source file.

With that said, it could be removed for the case without an extension, or even kept just for png / svg / pdf.

[PepinhoJp]

The compiler will now only suggest adding .typ if the .typ file exists. @MDLC01 thanks for your input.

[laurmaedje]

I think the implementation could still be simplified a bit, I've left a few comments to that effect.

I now also saw why the Hint<T> for SourceResult<T> impl wasn't in place yet, but I think with a few other changes I proposed, it won't be needed anymore anyway. Sorry for sending you down the wrong path with this!

[laurmaedje]

Thank you!