Add documentation

* `replace`
* `switch_case`
* `switch_to_uppercase`
* `switch_to_lowercase`

runtime/help/static-commands/replace.txt

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+`replace`
+
+Waits for another keypress, then replaces all
+selected characters with the pressed key.
+
+--- Examples ---
+
+'a' is replaced with 'e'.
+┌──────────────────────────┐  e  ┌──────────────────────────┐
+│ Do this, th[a]n do that. │ --> │ Do this, th[e]n do that. │
+└──────────────────────────┘     └──────────────────────────┘
+
+All instances of ',' are replaced with '.'.
+┌──────────────────────────────┐  .  ┌──────────────────────────────┐
+│ This sentence continues(,,,] │ --> │ This sentence continues(...] │
+└──────────────────────────────┘     └──────────────────────────────┘
+
+All instances of 'a' are replaced with 'e'.
+┌──────────────────────────────────┐  e  ┌──────────────────────────────────┐
+│ 1, th[a]n 2, th[a]n 3, th[a]n 4. │ --> │ 1, th[e]n 2, th[e]n 3, th[e]n 4. │
+└──────────────────────────────────┘     └──────────────────────────────────┘

runtime/help/static-commands/switch_case.txt

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+`switch_case`
+
+Toggles the case of all selected letters.
+
+--- Examples ---
+
+┌───────────────────────┐     ┌───────────────────────┐
+│ [t]his is a sentence. │ --> │ [T]his is a sentence. │
+└───────────────────────┘     └───────────────────────┘
+
+┌───────────────────────┐     ┌───────────────────────┐
+│ (tHIS] is a sentence. │ --> │ (This] is a sentence. │
+└───────────────────────┘     └───────────────────────┘

runtime/help/static-commands/switch_to_lowercase.txt

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+`switch_to_lowercase`
+
+Changes all selected letters to lowercase.
+
+--- Examples ---
+
+┌──────────────────────────────────┐     ┌──────────────────────────────────┐
+│ This (wOrD] should be lowercase. │ --> │ This (word] should be lowercase. │
+└──────────────────────────────────┘     └──────────────────────────────────┘

runtime/help/static-commands/switch_to_uppercase.txt

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+`switch_to_uppercase`
+
+Changes all selected letters to uppercase.
+
+--- Examples ---
+
+┌──────────────────────────────────┐     ┌──────────────────────────────────┐
+│ This (wOrD] should be uppercase. │ --> │ This (WORD] should be uppercase. │
+└──────────────────────────────────┘     └──────────────────────────────────┘