Add tailwindcss (#29357)

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>

Makefile

@@ -119,7 +119,7 @@ GO_TEST_PACKAGES ?= $(filter-out $(shell $(GO) list code.gitea.io/gitea/models/m
 FOMANTIC_WORK_DIR := web_src/fomantic
 
 WEBPACK_SOURCES := $(shell find web_src/js web_src/css -type f)
-WEBPACK_CONFIGS := webpack.config.js
+WEBPACK_CONFIGS := webpack.config.js tailwind.config.js
 WEBPACK_DEST := public/assets/js/index.js public/assets/css/index.css
 WEBPACK_DEST_ENTRIES := public/assets/js public/assets/css public/assets/fonts public/assets/img/webpack