We need to scan `.go` files for tailwind classes. Does not seem to
affect build time much luckily.

Fixes:
https://github.com/go-gitea/gitea/pull/29678#discussion_r1518448600

Verified via `rg tw-object-contain public/assets/css/index.css`.

---------

Co-authored-by: Giteabot <teabot@gitea.io>

Tailwind content is not going to appear in `web_src/css`,
`web_src/fomantic` or `web_src/svg` or the JSON templates, so we don't
need to have tailwind scan these directories which will speed up the
build.

---------

Co-authored-by: delvh <dev.lh@web.de>

Get rid of the `postcss@7` dependency by inlining this simple function.

As per discussion in https://github.com/go-gitea/gitea/pull/29423, I
think this is the right way that does not burden developers having to
think about CSS precedence which should be irrelevant with an atomic CSS
framework.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

This will get tailwindcss working on a basic level. It provides only the
utility classes, e.g. no tailwind base which we don't need because we
have our own CSS reset. Without the base, we also do not have their CSS
variables so a small amount of features do not work and I removed the
generated classes for them.

***Note for future developers: This currently uses a `tw-` prefix, so we
use it like `tw-p-3`.***

<details>
<summary>Currently added CSS, all false-positives</summary>

```
.\!visible{

    visibility: visible !important
}

.visible{

    visibility: visible
}

.invisible{

    visibility: hidden
}

.collapse{

    visibility: collapse
}

.static{

    position: static
}

.\!fixed{

    position: fixed !important
}

.absolute{

    position: absolute
}

.relative{

    position: relative
}

.sticky{

    position: sticky
}

.left-10{

    left: 2.5rem
}

.isolate{

    isolation: isolate
}

.float-right{

    float: right
}

.float-left{

    float: left
}

.mr-2{

    margin-right: 0.5rem
}

.mr-3{

    margin-right: 0.75rem
}

.\!block{

    display: block !important
}

.block{

    display: block
}

.inline-block{

    display: inline-block
}

.inline{

    display: inline
}

.flex{

    display: flex
}

.inline-flex{

    display: inline-flex
}

.\!table{

    display: table !important
}

.inline-table{

    display: inline-table
}

.table-caption{

    display: table-caption
}

.table-cell{

    display: table-cell
}

.table-column{

    display: table-column
}

.table-column-group{

    display: table-column-group
}

.table-footer-group{

    display: table-footer-group
}

.table-header-group{

    display: table-header-group
}

.table-row-group{

    display: table-row-group
}

.table-row{

    display: table-row
}

.flow-root{

    display: flow-root
}

.inline-grid{

    display: inline-grid
}

.contents{

    display: contents
}

.list-item{

    display: list-item
}

.\!hidden{

    display: none !important
}

.hidden{

    display: none
}

.flex-shrink{

    flex-shrink: 1
}

.shrink{

    flex-shrink: 1
}

.flex-grow{

    flex-grow: 1
}

.grow{

    flex-grow: 1
}

.border-collapse{

    border-collapse: collapse
}

.select-all{

    user-select: all
}

.resize{

    resize: both
}

.flex-wrap{

    flex-wrap: wrap
}

.overflow-visible{

    overflow: visible
}

.rounded{

    border-radius: 0.25rem
}

.border{

    border-width: 1px
}

.text-justify{

    text-align: justify
}

.uppercase{

    text-transform: uppercase
}

.lowercase{

    text-transform: lowercase
}

.capitalize{

    text-transform: capitalize
}

.italic{

    font-style: italic
}

.text-red{

    color: var(--color-red)
}

.text-shadow{

    color: var(--color-shadow)
}

.underline{

    text-decoration-line: underline
}

.overline{

    text-decoration-line: overline
}

.line-through{

    text-decoration-line: line-through
}

.outline{

    outline-style: solid
}

.ease-in{

    transition-timing-function: cubic-bezier(0.4, 0, 1, 1)
}

.ease-in-out{

    transition-timing-function: cubic-bezier(0.4, 0, 0.2, 1)
}

.ease-out{

    transition-timing-function: cubic-bezier(0, 0, 0.2, 1)
}
```

</details>

---------

Co-authored-by: Giteabot <teabot@gitea.io>