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/aymerick/douceur/css/declaration.go

@@ -0,0 +1,60 @@
+package css
+
+import "fmt"
+
+// Declaration represents a parsed style property
+type Declaration struct {
+	Property  string
+	Value     string
+	Important bool
+}
+
+// NewDeclaration instanciates a new Declaration
+func NewDeclaration() *Declaration {
+	return &Declaration{}
+}
+
+// Returns string representation of the Declaration
+func (decl *Declaration) String() string {
+	return decl.StringWithImportant(true)
+}
+
+// StringWithImportant returns string representation with optional !important part
+func (decl *Declaration) StringWithImportant(option bool) string {
+	result := fmt.Sprintf("%s: %s", decl.Property, decl.Value)
+
+	if option && decl.Important {
+		result += " !important"
+	}
+
+	result += ";"
+
+	return result
+}
+
+// Equal returns true if both Declarations are equals
+func (decl *Declaration) Equal(other *Declaration) bool {
+	return (decl.Property == other.Property) && (decl.Value == other.Value) && (decl.Important == other.Important)
+}
+
+//
+// DeclarationsByProperty
+//
+
+// DeclarationsByProperty represents sortable style declarations
+type DeclarationsByProperty []*Declaration
+
+// Implements sort.Interface
+func (declarations DeclarationsByProperty) Len() int {
+	return len(declarations)
+}
+
+// Implements sort.Interface
+func (declarations DeclarationsByProperty) Swap(i, j int) {
+	declarations[i], declarations[j] = declarations[j], declarations[i]
+}
+
+// Implements sort.Interface
+func (declarations DeclarationsByProperty) Less(i, j int) bool {
+	return declarations[i].Property < declarations[j].Property
+}