Fix no-raw-text crash on template literals without interpolations

[theRizwan]

Fixes #341.

report in no-raw-text read node.expressions[0].name unconditionally. A template literal with no
interpolations has an empty expressions array, so this threw and aborted the whole lint run
rather than reporting a lint error:

<View>{`hello`}</View>

TypeError: Cannot read properties of undefined (reading 'name')
Occurred while linting <input>:1
Rule: "react-native/no-raw-text"

A second, milder case: when the first expression is not an Identifier it has no .name, so
<View>{${this.props.text}}</View> reported Raw text (TemplateLiteral: undefined).

The change

Both cases now go through one helper:

Input	Before	After
{hello}	throws	Raw text (hello)
{${text}}	Raw text (TemplateLiteral: text)	unchanged
{${this.props.text}}	Raw text (TemplateLiteral: undefined)	Raw text (TemplateLiteral: this.props.text)

The no-interpolation case reports the literal text, which matches how a plain string expression
{'some text'} is already reported. The Identifier case is deliberately left alone so existing
messages and tests still hold.

context.getSourceCode() is used rather than context.sourceCode, since peerDependencies starts at
eslint@^3.17.0 and the modern accessor only exists from 8.40.

Tests

Two cases added to tests/lib/rules/no-raw-text.js, both of which fail on master:

• reverting only the lib/ change leaves 4 failing (RuleTester exercises each in both parser modes) — reproducing the TypeError and the undefined message
• with the fix, npm test is green: lint plus 133 passing

Happy to adjust the message wording for the no-interpolation case if you'd prefer something else —
reporting the literal text seemed most consistent with the existing string-expression behaviour, but
it is the one judgement call in here.