feat(textinput): do not block input on validation

This PR builds upon the excellent work in charmbracelet#167 and charmbracelet#114 and makes a
breaking change to the validation API.

Currently, validation will completely block text input if the Validate
function returns an error. This is now changed so the function no longer
blocks input if this is the case, thus handing this responsibility to
the clients.

This is helpful for cases where the user is requested to type an
existing system path, and the Validate function keeps asserting the
existence of the path. With the current implementation such a validation
is not possible.

For example:

    > /
    Err: nil

    > /t
    Err: /t: No such file or directory

    > /tm
    Err: /tm: No such file or directory

    > /tmp
    Err: nil

textinput/textinput.go

@@ -181,19 +181,14 @@ func (m *Model) SetValue(s string) {
 	// Clean up any special characters in the input provided by the
 	// caller. This avoids bugs due to e.g. tab characters and whatnot.
 	runes := m.san().Sanitize([]rune(s))
-	m.setValueInternal(runes)
+	err := m.validateIfDefined(string(runes))
+	m.setValueInternal(runes, err)
 }
 
-func (m *Model) setValueInternal(runes []rune) {
-	if m.Validate != nil {
-		if err := m.Validate(string(runes)); err != nil {
-			m.Err = err
-			return
-		}
-	}
+func (m *Model) setValueInternal(runes []rune, err error) {
+	m.Err = err
 
 	empty := len(m.value) == 0
-	m.Err = nil
 
 	if m.CharLimit > 0 && len(runes) > m.CharLimit {
 		m.value = runes[:m.CharLimit]
@@ -307,8 +302,6 @@ func (m *Model) insertRunesFromUserInput(v []rune) {
 	tail := make([]rune, len(tailSrc))
 	copy(tail, tailSrc)
 
-	oldPos := m.pos
-
 	// Insert pasted runes
 	for _, r := range paste {
 		head = append(head, r)
@@ -323,11 +316,8 @@ func (m *Model) insertRunesFromUserInput(v []rune) {
 
 	// Put it all back together
 	value := append(head, tail...)
-	m.setValueInternal(value)
-
-	if m.Err != nil {
-		m.pos = oldPos
-	}
+	inputErr := m.validateIfDefined(string(value))
+	m.setValueInternal(value, inputErr)
 }
 
 // If a max width is defined, perform some logic to treat the visible area
@@ -378,6 +368,7 @@ func (m *Model) handleOverflow() {
 // deleteBeforeCursor deletes all text before the cursor.
 func (m *Model) deleteBeforeCursor() {
 	m.value = m.value[m.pos:]
+	m.Err = m.validateIfDefined(string(m.value))
 	m.offset = 0
 	m.SetCursor(0)
 }
@@ -387,6 +378,7 @@ func (m *Model) deleteBeforeCursor() {
 // masked input.
 func (m *Model) deleteAfterCursor() {
 	m.value = m.value[:m.pos]
+	m.Err = m.validateIfDefined(string(m.value))
 	m.SetCursor(len(m.value))
 }
 
@@ -432,6 +424,7 @@ func (m *Model) deleteWordBackward() {
 	} else {
 		m.value = append(m.value[:m.pos], m.value[oldPos:]...)
 	}
+	m.Err = m.validateIfDefined(string(m.value))
 }
 
 // deleteWordForward deletes the word right to the cursor. If input is masked
@@ -471,6 +464,7 @@ func (m *Model) deleteWordForward() {
 	} else {
 		m.value = append(m.value[:oldPos], m.value[m.pos:]...)
 	}
+	m.Err = m.validateIfDefined(string(m.value))
 
 	m.SetCursor(oldPos)
 }
@@ -575,12 +569,12 @@ func (m Model) Update(msg tea.Msg) (Model, tea.Cmd) {
 	case tea.KeyMsg:
 		switch {
 		case key.Matches(msg, m.KeyMap.DeleteWordBackward):
-			m.Err = nil
 			m.deleteWordBackward()
 		case key.Matches(msg, m.KeyMap.DeleteCharacterBackward):
 			m.Err = nil
 			if len(m.value) > 0 {
 				m.value = append(m.value[:max(0, m.pos-1)], m.value[m.pos:]...)
+				m.Err = m.validateIfDefined(string(m.value))
 				if m.pos > 0 {
 					m.SetCursor(m.pos - 1)
 				}
@@ -597,13 +591,12 @@ func (m Model) Update(msg tea.Msg) (Model, tea.Cmd) {
 			if m.pos < len(m.value) {
 				m.SetCursor(m.pos + 1)
 			}
-		case key.Matches(msg, m.KeyMap.DeleteWordBackward):
-			m.deleteWordBackward()
 		case key.Matches(msg, m.KeyMap.LineStart):
 			m.CursorStart()
 		case key.Matches(msg, m.KeyMap.DeleteCharacterForward):
 			if len(m.value) > 0 && m.pos < len(m.value) {
 				m.value = append(m.value[:m.pos], m.value[m.pos+1:]...)
+				m.Err = m.validateIfDefined(string(m.value))
 			}
 		case key.Matches(msg, m.KeyMap.LineEnd):
 			m.CursorEnd()
@@ -884,3 +877,10 @@ func (m *Model) previousSuggestion() {
 		m.currentSuggestionIndex = len(m.matchedSuggestions) - 1
 	}
 }
+
+func (m Model) validateIfDefined(v string) error {
+	if m.Validate != nil {
+		return m.Validate(v)
+	}
+	return nil
+}