docs: any variables experiment

docs/docs/experiments/any_variables.md

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+---
+slug: /experiments/any-variables/
+---
+
+# Any Variables
+
+- Issue: [#1415](https://github.com/go-task/task/issues/1415)
+- Environment variable: `TASK_X_ANY_VARIABLES=1`
+- Breaks:
+  - Dynamically defined variables (using the `sh` keyword)
+
+Currently, Task only supports string variables. This experiment allows you to
+specify and use the following variable types:
+
+- `string`
+- `bool`
+- `int`
+- `float`
+- `array`
+- `map`
+
+This allows you to have a lot more flexibility in how you use variables in
+Task's templating engine. For example:
+
+Evaluating booleans:
+
+```yaml
+version: 3
+
+tasks:
+  foo:
+    vars:
+      BOOL: false
+    cmds:
+      - '{{ if .BOOL }}echo foo{{ end}}'
+```
+
+Arithmetic:
+
+```yaml
+version: 3
+
+tasks:
+  foo:
+    vars:
+      INT: 10
+      FLOAT: 3.14159
+    cmds:
+      - 'echo {{ add .INT .FLOAT }}'
+```
+
+Loops:
+
+```yaml
+version: 3
+
+tasks:
+  foo:
+    vars:
+      ARRAY: [1, 2, 3]
+    cmds:
+      - 'echo {{ range .ARRAY }}{{ . }}{{ end }}'
+```
+
+etc.
+
+## Migration
+
+Taskfiles with dynamically defined variables via the `sh` subkey will no longer
+work with this experiment enabled. In order to keep using dynamically defined
+variables, you will need to migrate your Taskfile to use the new syntax.
+
+Previously, you might have defined a dynamic variable like this:
+
+```yaml
+version: 3
+
+task:
+  foo:
+    vars:
+      CALCULATED_VAR:
+        sh: 'echo hello'
+    cmds:
+      - 'echo {{ .CALCULATED_VAR }}'
+```
+
+With this experiment enabled, you will need to remove the `sh` subkey and define
+your command as a string that begins with a `$`. This will instruct Task to
+interpret the string as a command instead of a literal value and the variable
+will be populated with the output of the command. For example:
+
+```yaml
+version: 3
+
+task:
+  foo:
+    vars:
+      CALCULATED_VAR: '$echo hello'
+    cmds:
+      - 'echo {{ .CALCULATED_VAR }}'
+```
+
+If your current Taskfile contains a string variable that begins with a `$`, you
+will now need to escape the `$` with a backslash (`\`) to stop Task from
+executing it as a command.