[stdlib] Use more idiomatic and performant implementation of isBlank

Also replace expect-actual with a common function because its implementation is no longer different in different platforms.

KT-65590

kotlin-native/runtime/src/main/kotlin/kotlin/text/Strings.kt

@@ -114,13 +114,6 @@ public actual fun String.replaceFirst(oldValue: String, newValue: String, ignore
     return if (index < 0) this else this.replaceRange(index, index + oldValue.length, newValue)
 }
 
-/**
- * Returns `true` if this string is empty or consists solely of whitespace characters.
- *
- * @sample samples.text.Strings.stringIsBlank
- */
-public actual fun CharSequence.isBlank(): Boolean = length == 0 || indices.all { this[it].isWhitespace() }
-
 /**
  * Returns the substring of this string starting at the [startIndex] and ending right before the [endIndex].
  *

libraries/stdlib/common/src/kotlin/TextH.kt

@@ -421,7 +421,6 @@ public expect fun String.endsWith(suffix: String, ignoreCase: Boolean = false):
 internal expect fun String.nativeIndexOf(ch: Char, fromIndex: Int): Int
 internal expect fun String.nativeLastIndexOf(ch: Char, fromIndex: Int): Int
 
-public expect fun CharSequence.isBlank(): Boolean
 /**
  * Returns `true` if the specified range in this char sequence is equal to the specified range in another char sequence.
  * @param thisOffset the start offset in this char sequence of the substring to compare.

libraries/stdlib/js/src/kotlin/text/stringsCode.kt

@@ -54,13 +54,6 @@ public fun String.matches(regex: String): Boolean {
     return result != null && result.size != 0
 }
 
-/**
- * Returns `true` if this string is empty or consists solely of whitespace characters.
- *
- * @sample samples.text.Strings.stringIsBlank
- */
-public actual fun CharSequence.isBlank(): Boolean = length == 0 || indices.all { this[it].isWhitespace() }
-
 /**
  * Returns `true` if this string is equal to [other], optionally ignoring character case.
  *

libraries/stdlib/jvm/src/kotlin/text/StringsJVM.kt

@@ -614,13 +614,6 @@ public actual fun CharSequence?.contentEquals(other: CharSequence?, ignoreCase:
 @kotlin.internal.InlineOnly
 public inline fun String.intern(): String = (this as java.lang.String).intern()
 
-/**
- * Returns `true` if this string is empty or consists solely of whitespace characters.
- *
- * @sample samples.text.Strings.stringIsBlank
- */
-public actual fun CharSequence.isBlank(): Boolean = length == 0 || indices.all { this[it].isWhitespace() }
-
 /**
  * Returns the index within this string that is offset from the given [index] by [codePointOffset] code points.
  */

libraries/stdlib/src/kotlin/text/Strings.kt

@@ -303,9 +303,12 @@ public inline fun CharSequence.isEmpty(): Boolean = length == 0
 @kotlin.internal.InlineOnly
 public inline fun CharSequence.isNotEmpty(): Boolean = length > 0
 
-// implemented differently in JVM and JS
-//public fun String.isBlank(): Boolean = length() == 0 || all { it.isWhitespace() }
-
+/**
+ * Returns `true` if this char sequence is empty or consists solely of whitespace characters.
+ *
+ * @sample samples.text.Strings.stringIsBlank
+ */
+public fun CharSequence.isBlank(): Boolean = all { it.isWhitespace() }
 
 /**
  * Returns `true` if this char sequence is not empty and contains some characters except whitespace characters.

libraries/stdlib/wasm/src/kotlin/text/StringsWasm.kt

@@ -499,13 +499,6 @@ public actual fun String.endsWith(suffix: String, ignoreCase: Boolean = false):
 
 // From stringsCode.kt
 
-/**
- * Returns `true` if this string is empty or consists solely of whitespace characters.
- *
- * @sample samples.text.Strings.stringIsBlank
- */
-public actual fun CharSequence.isBlank(): Boolean = length == 0 || indices.all { this[it].isWhitespace() }
-
 /**
  * Returns `true` if the specified range in this char sequence is equal to the specified range in another char sequence.
  * @param thisOffset the start offset in this char sequence of the substring to compare.