feat(compiler): support tokenizing escaped strings (#28055)

In order to support source mapping of templates, we need
to be able to tokenize the template in its original context.
When the template is defined inline as a JavaScript string
in a TS/JS source file, the tokenizer must be able to handle
string escape sequences, such as `\n` and `\"` as they
appear in the original source file.

This commit teaches the lexer how to unescape these
sequences, but only when the `escapedString` option is
set to true.  Otherwise there is no change to the tokenizing
behaviour.

PR Close #28055

packages/compiler/src/render3/view/template.ts

@@ -1580,6 +1580,30 @@ export interface ParseTemplateOptions {
    * The entire `source` string is parsed if this is not provided.
    * */
   range?: LexerRange;
+  /**
+   * If this text is stored in a JavaScript string, then we have to deal with escape sequences.
+   *
+   * **Example 1:**
+   *
+   * ```
+   * "abc\"def\nghi"
+   * ```
+   *
+   * - The `\"` must be converted to `"`.
+   * - The `\n` must be converted to a new line character in a token,
+   *   but it should not increment the current line for source mapping.
+   *
+   * **Example 2:**
+   *
+   * ```
+   * "abc\
+   *  def"
+   * ```
+   *
+   * The line continuation (`\` followed by a newline) should be removed from a token
+   * but the new line should increment the current line for source mapping.
+   */
+  escapedString?: boolean;
 }
 
 /**