Detect markdown line width, resolve relative URLs (#332)

~~this is semi-blocked by https://github.com/charmbracelet/glamour/pull/96, but behaviour isn't really worse than the previous behaviour (most links work, some are still broken)~~

#### testcase for link resolver
```
tea pr 332
tea checkout 332 && make install && tea pr 332
```

- [rel](./332)
- [abs](/gitea/tea/pulls/332)
- [full](https://gitea.com/gitea/tea/pulls/332)

Co-authored-by: Norwin Roosen <git@nroo.de>
Co-authored-by: 6543 <6543@obermui.de>
Reviewed-on: https://gitea.com/gitea/tea/pulls/332
Reviewed-by: 6543 <6543@obermui.de>
Reviewed-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: Norwin <noerw@noreply.gitea.io>
Co-committed-by: Norwin <noerw@noreply.gitea.io>

vendor/github.com/yuin/goldmark-emoji/definition/definition.go

@@ -0,0 +1,106 @@
+package definition
+
+// Emoji is a data structure that holds a single emoji.
+type Emoji struct {
+	// Name is a name of this emoji.
+	Name string
+
+	// ShortNames is a shorter representation of this emoji.
+	ShortNames []string
+
+	// Unicode is an unicode representation of this emoji.
+	Unicode []rune
+}
+
+// NewEmoji returns a new Emoji.
+func NewEmoji(name string, unicode []rune, shortNames ...string) Emoji {
+	if len(shortNames) == 0 {
+		panic("Emoji must have at leat 1 short name.")
+	}
+	if unicode == nil || len(unicode) == 0 {
+		unicode = []rune{0xFFFD}
+	}
+	return Emoji{
+		Name:       name,
+		ShortNames: shortNames,
+		Unicode:    unicode,
+	}
+}
+
+// IsUnicode returns true if this emoji is defined in unicode, otherwise false.
+func (em *Emoji) IsUnicode() bool {
+	return !(len(em.Unicode) == 1 && em.Unicode[0] == 0xFFFD)
+}
+
+// Emojis is a collection of emojis.
+type Emojis interface {
+	// Get returns (*Emoji, true) if found mapping associated with given short name, otherwise (nil, false).
+	Get(shortName string) (*Emoji, bool)
+
+	// Add adds new emojis to this collection.
+	Add(Emojis)
+
+	// Clone clones this collection.
+	Clone() Emojis
+}
+
+type emojis struct {
+	list     []Emoji
+	m        map[string]*Emoji
+	children []Emojis
+}
+
+// NewEmojis returns a new Emojis.
+func NewEmojis(es ...Emoji) Emojis {
+	m := &emojis{
+		list:     es,
+		m:        map[string]*Emoji{},
+		children: []Emojis{},
+	}
+	for i, _ := range es {
+		emoji := &m.list[i]
+		for _, s := range emoji.ShortNames {
+			m.m[s] = emoji
+		}
+	}
+	return m
+}
+
+func (m *emojis) Add(emojis Emojis) {
+	m.children = append(m.children, emojis)
+}
+
+func (m *emojis) Clone() Emojis {
+	es := &emojis{
+		list:     m.list,
+		m:        m.m,
+		children: make([]Emojis, len(m.children)),
+	}
+	copy(es.children, m.children)
+	return es
+}
+
+func (m *emojis) Get(shortName string) (*Emoji, bool) {
+	v, ok := m.m[shortName]
+	if ok {
+		return v, ok
+	}
+
+	for _, es := range m.children {
+		v, ok := es.Get(shortName)
+		if ok {
+			return v, ok
+		}
+	}
+	return nil, false
+}
+
+// EmojisOption sets options for Emojis.
+type EmojisOption func(Emojis)
+
+// WithEmojis is an EmojisOption that adds emojis to the Emojis.
+func WithEmojis(emojis ...Emoji) EmojisOption {
+	return func(m Emojis) {
+		m.Add(NewEmojis(emojis...))
+	}
+}