i18n: support reusing the same placeholder for ICU

[connorjclark]

It can be useful to reuse the same ICU variable names in a i18n string. For example:

/**
   * @description Text block that compares a local metric value to real user experiences.
   * @example {LCP} PH1
   * @example {500 ms} PH2
   * @example {400 ms} PH3
   * @example {40%} PH4
   */
  needsImprovementPoorDetailedCompare:
      'Your local {PH1} {PH2} needs improvement and is rated the same as {PH4} of real-user {PH1} experiences. However, the field data 75th percentile {PH1} {PH3} is poor.',

source

The above currently ends up with this in our ctc file: "message": "Your local $ICU_0$ $ICU_1$ needs improvement and is rated the same as $ICU_3$ of real-user {PH1} experiences. However, the field data 75th percentile {PH1} $ICU_2$ is good." (this is wrong).

We already support replacing multiple instances of a ICU variable: https://github.com/GoogleChrome/lighthouse/blob/c79628af9bdaa537a2abd1b34da922e28b81bd98/core/test/scripts/i18n/bake-ctc-to-lhl-test.js#L43C50-L60

But our collect strings step does not currently ever produce a ICU message string that reuses the same variable. For direct ICU (like the above example), it would just leave the {PH1} literal as-is for subsequent instances. For the custom format one, it would create a new variable. This PR fixes both of those cases.

internal links:

https://localizer.google.com/query/13584722