feat: add option to convert tabs into spaces

[fig-eater]

Adds an optional argument convert_tab_size to CLI and Markdown formatter options. The argument is optional the user may provide an integer in the range of [0, 255] to convert tabs into that number of spaces.

I just used replaceOwned to do this as that is what was already being done with turning \n to \n\n but this could be a point of improvement, changing points where we replace these with a alloc-less writer.

I was struggling run the executable for bbcodez to test this so I needed to make some additional fixes:
Fixes:

• Use createFile instead of openFile for creating the output file.
• Flush after writing a textElement, no output was being generated for text-only input.

bbcode:

this should show as a tab: "	"
this should show as a tab: `	`
this should show as a tab: [code]	[/code]
this should show as backslash t: "\t"
this should show as backslash t: `\t`
this should show as backslash t: [code]\t[/code]
escaped: \[code\]	[/code\]

generated markdown (4 space tabs):

this should show as a tab: "    "

this should show as a tab: `    `

this should show as a tab: `    `

this should show as backslash t: "\t"

this should show as backslash t: `\t`

this should show as backslash t: `\t`

escaped: [code]    [/code]

Closes #1

[fig-eater]

For changes required on the gdzig side of things, we would just need to add this argument to the renderDocument call in docs.zig:
https://github.com/gdzig/gdzig/blob/9ec99bf4344cec830d509124821a96f68f3c5e3c/gdzig_bindgen/Context/docs.zig#L257

    try bbcodez.fmt.md.renderDocument(allocator, doc, &output.writer, .{
        .write_element_fn = writeElement,
        .user_data = @ptrCast(@constCast(&doc_ctx)),
        .convert_tab_size = 4, // new, may want to put as a const at the top of file.
    });

[deevus]

Nice one. Thanks!