feat: Add ui.theme() and theme argument to page_*() functions

[gadenbuie]

Closes #1270

[cpsievert]

I think we should check elif isinstance(theme, Tagifiable):?

[cpsievert]

Could we do this check on __init__? Could be confusing that a "bad" them can be constructed, but then later throws an error (and probably leads to a confusing stack trace)

[cpsievert]

Probably worth commenting that Tag | TagList are here to support the ui.include_css() case

[cpsievert]

I feel uneasy about having an exported shiny.ui.theme() that what is mainly a advanced user / developer facing API.

Also, I can't help but feel like Theme/theme naming is a bit of a misnomer, especially if we're thinking about this as a user facing API. In that case, maybe something like bootstrap_css() is better?

In fact, I hate to suggest this, but maybe we don't need a theme argument at all for this use case. Could we not have a bootstrap_css() that could be dropped anywhere in the UI? Implemented roughly like this?

__all__ = (
    "bootstrap_css",
)

def bootstrap_css(path: Path | str, check_version: bool = True) -> list[Tag | HTMLDependency]:
    # Snoop the version of the Bootstrap CSS file
    if check_version:
        check_bootstrap_version(path)

    return [
        ui.include_css(path),
        HTMLDependency(
            name="bootstrap-css", version=bootstrap_version
        ),
    ]

def check_bootstrap_version(path: Path | str):
    file_path = check_path(path)
    content = read_utf8(file_path)
    # TODO: extract version from Bootstrap's banner, then warn/err if not compatible, and
    # also warn if the version wasn't found?

def bootstrap_deps() -> list[HTMLDependency]:
    return [
        jquery_deps(),
        HTMLDependency(
            name="bootstrap-js",
            version=bootstrap_version,
            source={"package": "shiny", "subdir": "www/shared/bootstrap/"},
            script={"src": "bootstrap.bundle.min.js"},
            meta={"name": "viewport", "content": "width=device-width, initial-scale=1"},
        ),
        HTMLDependency(
            name="bootstrap-css",
            version=bootstrap_version,
            source={"package": "shiny", "subdir": "www/shared/bootstrap/"},
            stylesheet={"href": "bootstrap.min.css"}
        )
    ]

[gadenbuie]

Would you mind moving this comment to the design issue?

[cpsievert]

Two things to note about this approach:

1. We'd (attempt to) check the bootstrap version automatically. As long as Bootstrap isn't getting compiled in a weird way, this should work.

2. We'd (intentionally) drop the replace = "all" feature, but we could bring it back with a bootstrap_js() if need be down the road. In general though, the thought of encouraging users to customize the JS gives me the heebie jeebies.

[cpsievert]

Oh sorry, just saw your comment, yes will do.