Add 'Remove Blank Lines' and 'Keep Substring Matching' text processors

- Remove Blank Lines: filters out any lines that are empty or contain
  only whitespace
- Keep Substring Matching: prompts for a regex via FilterWithArg, then
  replaces each line with its first matching substring, dropping
  non-matching lines

Co-authored-by: Shelley <shelley@exe.dev>

textfilters.go

@@ -7,6 +7,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"regexp"
 	"strconv"
 	"strings"
 
@@ -273,6 +274,47 @@ var TextFilters = map[string]TextProcessor{
 		},
 	},
 
+	"remove-blank-lines": {
+		Label: "Lines: Remove Blank Lines",
+		Filter: func(ctx context.Context, input string) (resp TextFilterResponse, err error) {
+			var dst []string
+			scnr := bufio.NewScanner(strings.NewReader(input))
+			for scnr.Scan() {
+				line := scnr.Text()
+				if strings.TrimSpace(line) != "" {
+					dst = append(dst, line)
+				}
+			}
+			return TextFilterResponse{Output: strings.Join(dst, "\n")}, nil
+		},
+	},
+
+	"keep-substring-matching": {
+		Label:       "Lines: Keep Substring Matching…",
+		Description: "Prompts for a regular expression, then replaces each line with the first matching substring, removing non-matching lines.",
+		FilterWithArg: &FilterWithArg{
+			Label: "Regular Expression",
+			OnConfirm: func(ctx context.Context, prompt string) (filter TextFilter, err error) {
+				re, err := regexp.Compile(prompt)
+				if err != nil {
+					return nil, err
+				}
+
+				return func(ctx context.Context, input string) (resp TextFilterResponse, err error) {
+					var dst []string
+					scnr := bufio.NewScanner(strings.NewReader(input))
+					for scnr.Scan() {
+						line := scnr.Text()
+						if match := re.FindString(line); match != "" {
+							dst = append(dst, match)
+						}
+					}
+					return TextFilterResponse{Output: strings.Join(dst, "\n")}, nil
+				}, nil
+			},
+		},
+	},
+
 	"ucl-replace": {
 		Label:       "UCL: Replace",
 		Description: "Evaluates the input as a UCL expression and replaces it with the result.",